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  • 1.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Patient Dose Computation2014In: Comprehensive Biomedical Physics: Volume 9: Radiation Therapy Physics and Treatment Optimization / [ed] Anders Brahme, Amsterdam: Elsevier, 2014, p. 235-247Chapter in book (Refereed)
    Abstract [en]

    Various dose calculation methods have been proposed to serve the needs in treatment planning of radiotherapy. Common to these are that they need a patient model to describe the interaction properties of the irradiated tissues, and a sufficiently accurate description of the incident radiation. This chapter starts with a brief review of the contexts in which patient dose calculations may serve, followed by a description of common methods for patient modelling and beam characterization. The focus is on external beam photon, but also partly covers particle beams like electrons and protons. The last section describes common approaches of varying complexity for dose calculations ranging from simple factor based models, more elaborate pencil and point kernel models, and finally summarizes some aspects of Monte Carlo and grid based methods.

  • 2.
    Ahnesjö, Anders
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Section of Medical Physics.
    Eklund, Karin
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Section of Medical Physics. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Rikner, Göran
    Rönnqvist, Camilla
    Grusell, Erik
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Section of Medical Physics.
    Detector response modeling2009Patent (Other (popular science, discussion, etc.))
    Abstract [en]

    A detector response correction arrangement and method is proposed for online determination of correction factors for arbitrary positions from arbitrary incident fluence distributions. As modern radiotherapy utilizes more of the available degrees of freedom of radiation machines, dosimetry has to be able to present reliable measurements for all these degrees of freedom. To determine correction factors online during measurement, Monte Carlo technique is used to precalculate fluence pencil kernels from a monodirectional beam to fully describe the particle fluence in an irradiated medium. Assuming that the particle fluence is not significantly altered by the introduction of a small detector volume, the fluence pencil kernels (212) can be integrated (214), and correction factors (216) determined, e.g. by Cavity Theory, in different positions for the detector material.

  • 3.
    Ahnesjö, Anders
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    Hårdemark, Björn
    Isacsson, Ulf
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    Montelius, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    The IMRT information process-mastering the degrees of freedom in external beam therapy2006In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 13, p. R381-402Article in journal (Refereed)
    Abstract [en]

    The techniques and procedures for intensity-modulated radiation therapy (IMRT) are reviewed in the context of the information process central to treatment planning and delivery of IMRT. A presentation is given of the evolution of the information based radiotherapy workflow and dose delivery techniques, as well as the volume and planning concepts for relating the dose information to image based patient representations. The formulation of the dose shaping process as an optimization problem is described. The different steps in the calculation flow for determination of machine parameters for dose delivery are described starting from the formulation of optimization objectives over dose calculation to optimization procedures. Finally, the main elements of the quality assurance procedure necessary for implementing IMRT clinically are reviewed.

  • 4.
    Ahnesjö, Anders
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    van Veelen, Bob
    Elekta Brachytherapy, NL-3905 TH Veenendaal, Netherlands..
    Tedgren, Asa Carlsson
    Linkoping Univ, Fac Hlth Sci, Dept Med & Hlth Sci IMH, Radiat Phys, SE-58185 Linkoping, Sweden.;Karolinska Univ Hosp, Dept Med Phys, Sect Radiotherapy Phys & Engn, SE-17176 Stockholm, Sweden..
    Collapsed cone dose calculations for heterogeneous tissues in brachytherapy using primary and scatter separation source data2017In: Computer Methods and Programs in Biomedicine, ISSN 0169-2607, E-ISSN 1872-7565, Vol. 139, p. 17-29Article in journal (Refereed)
    Abstract [en]

    Background and Objective: Brachytherapy is a form of radiation therapy using sealed radiation sources inserted within or in the vicinity of the tumor of, e.g., gynecological, prostate or head and neck cancers. Accurate dose calculation is a crucial part of the treatment planning. Several reviews have called for clinical software with model-based algorithms that better take into account the effects of patient individual distribution of tissues, source-channel and shielding attenuation than the commonly employed TG-43 formalism which simply map homogeneous water dose distributions onto the patient. In this paper we give a comprehensive and thorough derivation of such an algorithm based on collapsed cone point-kernel superposition, and describe details of its implementation into a commercial treatment planning system for clinical use. Methods: A brachytherapy version of the collapsed-cone algorithm using analytical raytraces of the primary photon radiation followed by successive scattering dose calculation for once and multiply scattered photons is described in detail, including derivation of the corresponding set of recursive equations for energy transport along cone axes/transport lines and the coupling to clinical source modeling. Specific implementation issues for setting up of the calculation grid, handling of intravoxel gradients and voxels partly containing non patient applicator material are given. Results: Sample runs for two clinical cases are shown, one being a gynecological application with a tungsten-shielded applicator and one a breast implant. These two cases demonstrate the impact of improved dose calculation versus TG-43 formalism. Conclusions: Use of model-based dose calculation algorithms for brachytherapy taking the three-dimensional treatment geometry into account increases the dosimetric accuracy in planning and follow up of treatments. The comprehensive description and derivations provided gives a rigid background for further clinical, educational and research applications.

  • 5.
    Ahnesjö, Anders
    et al.
    Uppsala University, Medicinska vetenskapsområdet, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology. Onkologi.
    Weber, Lars
    Murman, Anders
    Saxner, Mikael
    Thorslund, Ingvar
    Traneus, Erik
    Beam modeling and verification of a photon beam multisource model.2005In: Medical Physics, ISSN 0094-2405, Vol. 32, no 6, p. 1722-37Article in journal (Refereed)
    Abstract [en]

    Dose calculations for treatment planning of photon beam radiotherapy require a model of the beam to drive the dose calculation models. The beam shaping process involves scattering and filtering that yield radiation components which vary with collimator settings. The necessity to model these components has motivated the development of multisource beam models. We describe and evaluate clinical photon beam modeling based on multisource models, including lateral beam quality variations. The evaluation is based on user data for a pencil kernel algorithm and a point kernel algorithm (collapsed cone) used in the clinical treatment planning systems Helax-TMS and Nucletron-Oncentra. The pencil kernel implementations treat the beam spectrum as lateral invariant while the collapsed cone involves off axis softening of the spectrum. Both algorithms include modeling of head scatter components. The parameters of the beam model are derived from measured beam data in a semiautomatic process called RDH (radiation data handling) that, in sequential steps, minimizes the deviations in calculated dose versus the measured data. The RDH procedure is reviewed and the results of processing data from a large number of treatment units are analyzed for the two dose calculation algorithms. The results for both algorithms are similar, with slightly better results for the collapsed cone implementations. For open beams, 87% of the machines have maximum errors less than 2.5%. For wedged beams the errors were found to increase with increasing wedge angle. Internal, motorized wedges did yield slightly larger errors than external wedges. These results reflect the increased complexity, both experimentally and computationally, when wedges are used compared to open beams.

  • 6.
    Almhagen, Erik
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. The Skandion Clinic, Uppsala, Sweden.
    Boersma, David J.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. ACMIT Gmbh, A-2700 Wiener Neustadt, Austria.
    Nyström, H.
    Skandion Clin, Uppsala, Sweden;DCPT, Aarhus, Denmark.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    A beam model for focused proton pencil beams2018In: Physica medica (Testo stampato), ISSN 1120-1797, E-ISSN 1724-191X, Vol. 52, p. 27-32Article in journal (Refereed)
    Abstract [en]

    Introduction: We present a beam model for Monte Carlo simulations of the IBA pencil beam scanning dedicated nozzle installed at the Skandion Clinic. Within the nozzle, apart from entrance and exit windows and the two ion chambers, the beam traverses vacuum, allowing for a beam that is convergent downstream of the nozzle exit. Materials and methods: We model the angular, spatial and energy distributions of the beam phase space at the nozzle exit with single Gaussians, controlled by seven energy dependent parameters. The parameters were determined from measured profiles and depth dose distributions. Verification of the beam model was done by comparing measured and GATE acquired relative dose distributions, using plan specific log files from the machine to specify beam spot positions and energy. Results: GATE-based simulations with the acquired beam model could accurately reproduce the measured data. The gamma index analysis comparing simulated and measured dose distributions resulted in > 95% global gamma index pass rates (3%/2 mm) for all depths. Conclusion: The developed beam model was found to be sufficiently accurate for use with GATE e.g. for applications in quality assurance (QA) or patient motion studies with the IBA pencil beam scanning dedicated nozzles.

  • 7. Andersson, Karin M.
    et al.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Dahlgren, Christina Vallhagen
    Evaluation of a metal artifact reduction algorithm in CT studies used for proton radiotherapy treatment planning2014In: Journal of Applied Clinical Medical Physics, ISSN 1526-9914, E-ISSN 1526-9914, Vol. 15, no 5, p. 112-119Article in journal (Refereed)
    Abstract [en]

    Metal objects in the body such as hip prostheses cause artifacts in CT images. When CT images degraded by artifacts are used for treatment planning of radiotherapy, the artifacts can yield inaccurate dose calculations and, for particle beams, erroneous penetration depths. A metal artifact reduction software (O-MAR) installed on a Philips Brilliance Big Bore CT has been tested for applications in treatment planning of proton radiotherapy. Hip prostheses mounted in a water phantom were used as test objects. Images without metal objects were acquired and used as reference data for the analysis of artifact-affected regions outside of the metal objects in both the O-MAR corrected and the uncorrected images. Water equivalent thicknesses (WET) based on proton stopping power data were calculated to quantify differences in the calculated proton beam penetration for the different image sets. The WET to a selected point of interest between the hip prostheses was calculated for several beam directions of clinical relevance. The results show that the calculated differences in WET relative to the reference case were decreased when the O-MAR algorithm was applied. WET differences up to 2.0 cm were seen in the uncorrected case while, for the O-MAR corrected case, the maximum difference was decreased to 0.4 cm. The O-MAR algorithm can significantly improve the accuracy in proton range calculations. However, there are some residual effects, and the use of proton beam directions along artifact streaks should only be used with caution and appropriate margins.

  • 8.
    Andersson, Karin M.
    et al.
    Skand Clin, Uppsala, Sweden; Örebro Univ, Sch Hlth & Med Sci, Örebro, Sweden.
    Dahlgren, Christina Vallhagen
    Skand Clin, Uppsala, Sweden.
    Reizenstein, Johan
    Örebro Univ, Fac Med & Hlth, Dept Oncol, Örebro, Sweden.
    Cao, Yang
    Örebro Univ, Sch Med Sci, Clin Epidemiol & Biostat, Örebro, Sweden; Karolinska Inst, Inst Environm Med, Unit Biostat, Stockholm, Sweden.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Thunberg, Per
    Örebro Univ, Fac Med & Hlth, Dept Med Phys, Örebro, Sweden.
    Evaluation of two commercial CT metal artifact reduction algorithms for use in proton radiotherapy treatment planning in the head and neck area2018In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 45, no 10, p. 4329-4344Article in journal (Refereed)
    Abstract [en]

    Purpose: To evaluate two commercial CT metal artifact reduction (MAR) algorithms for use in proton treatment planning in the head and neck (H&N) area.

    Methods: An anthropomorphic head phantom with removable metallic implants (dental fillings or neck implant) was CT-scanned to evaluate the O-MAR (Philips) and the iMAR (Siemens) algorithms. Reference images were acquired without any metallic implants in place. Water equivalent thickness (WET) was calculated for different path directions and compared between image sets. Images were also evaluated for use in proton treatment planning for parotid, tonsil, tongue base, and neck node targets. The beams were arranged so as to not traverse any metal prior to the target, enabling evaluation of the impact on dose calculation accuracy from artifacts surrounding the metal volume. Plans were compared based on analysis (1 mm distance-to-agreement/1% difference in local dose) and dose volume histogram metrics for targets and organs at risk (OARs). Visual grading evaluation of 30 dental implant patient MAR images was performed by three radiation oncologists.

    Results: In the dental fillings images, WET along a low-density streak was reduced from -17.0 to -4.3 mm with O-MAR and from -16.1 mm to -2.3 mm with iMAR, while for other directions the deviations were increased or approximately unchanged when the MAR algorithms were used. For the neck implant images, WET was generally reduced with MAR but residual deviations remained (of up to -2.3 mm with O-MAR and of up to -1.5 mm with iMAR). The analysis comparing proton dose distributions for uncorrected/MAR plans and corresponding reference plans showed passing rates >98% of the voxels for all phantom plans. However, substantial dose differences were seen in areas of most severe artifacts ( passing rates of down to 89% for some cases). MAR reduced the deviations in some cases, but not for all plans. For a single patient case dosimetrically evaluated, minor dose differences were seen between the uncorrected and MAR plans ( passing rate approximately 97%). The visual grading of patient images showed that MAR significantly improved image quality (P < 0.001).

    Conclusions: O-MAR and iMAR significantly improved image quality in terms of anatomical visualization for target and OAR delineation in dental implant patient images. WET calculations along several directions, all outside the metallic regions, showed that both uncorrected and MAR images contained metal artifacts which could potentially lead to unacceptable errors in proton treatment planning. WET was reduced by MAR in some areas, while increased or unchanged deviations were seen for other path directions. The proton treatment plans created for the phantom images showed overall acceptable dose distributions differences when compared to the reference cases, both for the uncorrected and MAR images. However, substantial dose distribution differences in the areas of most severe artifacts were seen for some plans, which were reduced by MAR in some cases but not all. In conclusion, MAR could be beneficial to use for proton treatment planning; however, case-by-case evaluations of the metal artifact-degraded images are always recommended.

  • 9.
    Bäckström, Gloria
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Galassi, M. E.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Fernandez-Varea, J. M.
    Track structure of protons and other light ions in liquid water: Applications of the LIonTrack code at the nanometer scale2013In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 40, no 6, p. 064101-Article in journal (Refereed)
    Abstract [en]

    Purpose: The LIonTrack (Light Ion Track) Monte Carlo (MC) code for the simulation of H+, He2+, and other light ions in liquid water is presented together with the results of a novel investigation of energy-deposition site properties from single ion tracks. Methods: The continuum distorted-wave formalism with the eikonal initial state approximation (CDW-EIS) is employed to generate the initial energy and angle of the electrons emitted in ionizing collisions of the ions with H2O molecules. The model of Dingfelder et al. ["Electron inelastic-scattering cross sections in liquid water," Radiat. Phys. Chem. 53, 1-18 (1998); " Comparisons of calculations with PARTRAC and NOREC: Transport of electrons in liquid water," Radiat. Res. 169, 584-594 (2008)] is linked to the general-purpose MC code PENELOPE/penEasy to simulate the inelastic interactions of the secondary electrons in liquid water. In this way, the extended PENELOPE/penEasy code may provide an improved description of the 3D distribution of energy deposits (EDs), making it suitable for applications at the micrometer and nanometer scales. Results: Single-ionization cross sections calculated with the ab initio CDW-EIS formalism are compared to available experimental values, some of them reported very recently, and the theoretical electronic stopping powers are benchmarked against those recommended by the ICRU. The authors also analyze distinct aspects of the spatial patterns of EDs, such as the frequency of nearest-neighbor distances for various radiation qualities, and the variation of the mean specific energy imparted in nanoscopic targets located around the track. For 1 MeV/u particles, the C6+ ions generate about 15 times more clusters of six EDs within an ED distance of 3 nm than H+. Conclusions: On average clusters of two to three EDs for 1 MeV/u H+ and clusters of four to five EDs for 1 MeV/u C6+ could be expected for a modeling double strand break distance of 3.4 nm.

  • 10.
    Dahlgren, Christina Vallhagen
    et al.
    Uppsala University, Medicinska vetenskapsområdet, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology. Onkologi.
    Eilertsen, Karsten
    Jörgensen, Trude Dahl
    Ahnesjö, Anders
    Uppsala University, Medicinska vetenskapsområdet, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology. Onkologi.
    Portal dose image verification: the collapsed cone superposition method applied with different electronic portal imaging devices.2006In: Physics in Medicine and Biology, ISSN 0031-9155, Vol. 51, no 2, p. 335-49Article in journal (Refereed)
    Abstract [en]

    Two different commercial electronic portal imaging devices (EPIDs), one based on a liquid ion chamber matrix and the other based on a fluoroscopic CCD camera, were used to acquire readings that, through a calibration procedure, provided images proportional to the absolute dose to a virtual water slab located at the EPID plane. The transformation of the matrix ion chamber image into a portal dose image (PDI) was based on a published relationship between dose rate and ionization current. For the fluoroscopic CCD-camera-based system, the transformation was based on a deconvolution with a radial light scatter kernel. Local response variations were corrected in the images from both systems using open field fluence maps. The acquired PDIs were compared with PDIs calculated with the collapsed cone superposition method for a three-dimensional detector model in water equivalent buildup material. The calculation model was based on the beam modelling and geometrical description of the treatment unit and energy used for treatment planning in a kernel-based system. The validity of the calculation method was evaluated for several field shapes and thicknesses of patient phantoms for the matrix ion chamber at 6 MV x-rays and for the camera-based EPID at 6 and 15 MV x-rays. The agreement between predicted and measured PDIs was evaluated with dose comparisons at points of interest and gamma index calculations. The average area failing the passing criteria in dose and position deviation was analysed to validate the performance of the method. For the matrix ion chamber on average an area less than 1% fails the passing criteria of 3 mm and 3%. For the camera-based EPID, the average area is 7% and 1% for 6 and 15 MV, respectively. The overall agreement centrally in the fields was 0.1 +/- 1.6% (1 sd) for the camera-based EPID and -0.1 +/- 1.6% (1 sd) for the matrix ion chamber. Thus, an absolute dose calibrated EPID could validate the delivered dose to the patient by comparing a calculated and a measured PDI.

  • 11. Das, Indra J.
    et al.
    Cheng, Chee-Wai
    Watts, Ronald J.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Oncology.
    Gibbons, John
    Li, X. Allen
    Lowenstein, Jessica
    Mitra, Raj K.
    Simon, William E.
    Zhu, Timothy C.
    Accelerator beam data commissioning equipment and procedures: report of the TG-106 of the Therapy Physics Committee of the AAPM2008In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 35, no 9, p. 4186-4215Article, review/survey (Refereed)
    Abstract [en]

    For commissioning a linear accelerator for clinical use, medical physicists are faced with many challenges including the need for precision, a variety of testing methods, data validation, the lack of standards, and time constraints. Since commissioning beam data are treated as a reference and ultimately used by treatment planning systems, it is vitally important that the collected data are of the highest quality to avoid dosimetric and patient treatment errors that may subsequently lead to a poor radiation outcome. Beam data commissioning should be performed with appropriate knowledge and proper tools and should be independent of the person collecting the data. To achieve this goal, Task Group 106 (TG-106) of the Therapy Physics Committee of the American Association of Physicists in Medicine was formed to review the practical aspects as well as the physics of linear accelerator commissioning. The report provides guidelines and recommendations on the proper selection of phantoms and detectors, setting up of a phantom for data acquisition (both scanning and no-scanning data), procedures for acquiring specific photon and electron beam parameters and methods to reduce measurement errors (<1%), beam data processing and detector size convolution for accurate profiles. The TG-106 also provides a brief.discussion on the emerging trend in Monte Carlo simulation techniques in photon and electron beam commissioning. The procedures described in this report should assist a qualified medical physicist in either measuring a complete set of beam data, or in verifying a subset of data before initial use or for periodic quality assurance measurements. By combining practical experience with theoretical discussion, this document sets a new standard for beam data commissioning.

  • 12. Das, Indra J.
    et al.
    Ding, George X.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Oncology.
    Small fields: nonequilibrium radiation dosimetry2008In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 35, no 1, p. 206-215Article, review/survey (Refereed)
    Abstract [en]

    Advances in radiation treatment with beamlet-based intensity modulation, image-guided radiation therapy, and stereotactic radiosurgery (including specialized equipments like CyberKnife, Gamma Knife, tomotherapy, and high-resolution multileaf collimating systems) have resulted in the use of reduced treatment fields to a subcentimeter scale. Compared to the traditional radiotherapy with fields > or =4 x 4 cm2, this can result in significant uncertainty in the accuracy of clinical dosimetry. The dosimetry of small fields is challenging due to nonequilibrium conditions created as a consequence of the secondary electron track lengths and the source size projected through the collimating system that are comparable to the treatment field size. It is further complicated by the prolonged electron tracks in the presence of low-density inhomogeneities. Also, radiation detectors introduced into such fields usually perturb the level of disequilibrium. Hence, the dosimetric accuracy previously achieved for standard radiotherapy applications is at risk for both absolute and relative dose determination. This article summarizes the present knowledge and gives an insight into the future procedures to handle the nonequilibrium radiation dosimetry problems. It is anticipated that new miniature detectors with controlled perturbations and corrections will be available to meet the demand for accurate measurements. It is also expected that the Monte Carlo techniques will increasingly be used in assessing the accuracy, verification, and calculation of dose, and will aid perturbation calculations of detectors used in small and highly conformal radiation beams.

  • 13.
    Eklund, Karin
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Modeling silicon diode dose response factors for small photon fields2010In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 55, no 24, p. 7411-7423Article in journal (Refereed)
    Abstract [en]

    The dosimetry of small fields is important for the use of high resolution photon radiotherapy. Silicon diodes yield a high signal from a small detecting volume which makes them suitable for use in small fields and high dose gradients. Unshielded diodes used in large fields are known to give a varying dose response depending on the proportion of low energy scattered photons in the field. Response variations in small fields can be caused by both spectral variations, and disturbances of the local level of lateral electron equilibrium. We present a model that includes the effects from lack of charged particle equilibrium. The local spectra are calculated by use of fluence pencil kernels and divided into a low and a high energy component. The low energy part is treated with large cavity theory and the high energy part with the Spencer-Attix small cavity theory. Monte Carlo-derived correction factors are used to account for both the local level of electron equilibrium in the field, and deviations from this level in the silicon disk cavity. Results for field sizes ranging from 0.5 × 0.5 to 20 × 20 cm2 are compared to data from full Monte Carlo simulations and measurements. The achieved dose response accuracy is for the smallest fields 1-2%, and for larger fields 0.5%. Spectral variations were of little importance for the small field response, implying that volume averaging, and to some extent interface transient effects, are of importance for use of unshielded diodes in non-equilibrium conditions. The results indicate that diodes should preferably be designed to have the thin layer of active volume padded in between inactive layers of the silicon base material.

  • 14.
    Eklund, Karin
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Section of Medical Physics. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Section of Medical Physics. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Modeling silicon diode energy response factors for use in therapeutic photon beams2009In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 54, no 20, p. 6135-6150Article in journal (Refereed)
    Abstract [en]

    Silicon diodes have good spatial resolution, which makes them advantageous over ionization chambers for dosimetry in fields with high dose gradients. However, silicon diodes overrespond to low-energy photons, that are more abundant in scatter which increase with large fields and larger depths. We present a cavity-theory-based model for a general response function for silicon detectors at arbitrary positions within photon fields. The model uses photon and electron spectra calculated from fluence pencil kernels. The incident photons are treated according to their energy through a bipartition of the primary beam photon spectrum into low- and high-energy components. Primary electrons from the high-energy component are treated according to Spencer–Attix cavity theory. Low-energy primary photons together with all scattered photons are treated according to large cavity theory supplemented with an energy-dependent factor K(E) to compensate for energy variations in the electron equilibrium. The depth variation of the response for an unshielded silicon detector has been calculated for 5 × 5 cm2, 10 × 10 cm2 and 20 × 20 cm2 fields in 6 and 15 MV beams and compared with measurements showing that our model calculates response factors with deviations less than 0.6%. An alternative method is also proposed, where we show that one can use a correlation with the scatter factor to determine the detector response of silicon diodes with an error of less than 3% in 6 MV and 15 MV photon beams.

  • 15. Enger, Shirin A.
    et al.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics.
    Verhaegen, Frank
    Beaulieu, Luc
    Dose to tissue medium or water cavities as surrogate for the dose to cell nuclei at brachytherapy photon energies2012In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 14, p. 4489-4500Article in journal (Refereed)
    Abstract [en]

    It has been suggested that modern dose calculation algorithms should be able to report absorbed dose both as dose to the local medium, D-m,D-m, and as dose to a water cavity embedded in the medium, D-w,D-m, using conversion factors from cavity theory. Assuming that the cell nucleus with its DNA content is the most important target for biological response, the aim of this study is to investigate, by means of Monte Carlo (MC) simulations, the relationship of the dose to a cell nucleus in a medium, D-n,D-m, to D-m,D-m and D-w,D-m, for different combinations of cell nucleus compositions and tissue media for different photon energies used in brachytherapy. As D-n,D-m is very impractical to calculate directly for routine treatment planning, while D-m,D-m and D-w,D-m are much easier to obtain, the questions arise which one of these quantities is the best surrogate for D-n,D-m and which cavity theory assumptions should one use for its estimate. The Geant4.9.4 MC code was used to calculate D-m,D-m, D-w,D-m and D-n,D-m for photon energies from 20 (representing the lower energy end of brachytherapy for Pd-103 or I-125) to 300 keV (close to the mean energy of Ir-192) and for the tissue media adipose, breast, prostate and muscle. To simulate the cell and its nucleus, concentric spherical cavities were placed inside a cubic phantom (10 x 10 x 10 mm(3)). The diameter of the simulated nuclei was set to 14 mu m. For each tissue medium, three different setups were simulated; (a) D-n,D-m was calculated with nuclei embedded in tissues (MC-D-n,D-m). Four different published elemental compositions of cell nuclei were used. (b) D-w,D-m was calculated with MC (MC-D-w,D-m) and compared with large cavity theory calculated D-w,D-m (LCT-D-w,D-m), and small cavity theory calculated D-w,D-m (SCT-D-w,D-m). (c) D-m,D-m was calculated with MC (MC-D-m,D-m). MC-D-w,D-m is a good substitute for MC-D-n,D-m for all photon energies and for all simulated nucleus compositions and tissue types. SCT-D-w,D-m can be used for most energies in brachytherapy, while LCT-D-w,D-m should only be considered for source spectra well below 50 keV, since contributions to the absorbed dose inside the nucleus to a large degree stem from electrons released in the surrounding medium. MC-D-m,D-m is not an appropriate substitute for MC-D-n,D-m for the lowest photon energies for adipose and breast tissues. The ratio of MC-D-m,D-m to MC-D-n,D-m for adipose and breast tissue deviates from unity by 34% and 15% respectively for the lowest photon energy (20 keV), whereas the ratio is close to unity for higher energies. For prostate and muscle tissue MC-D-m,D-m is a good substitute for MC-D-n,D-m. However, for all photon energies and tissue types the nucleus composition with the highest hydrogen content behaves differently than other compositions. Elemental compositions of the tissue and nuclei affect considerably the absorbed dose to the cell nuclei for brachytherapy sources, in particular those at the low-energy end of the spectrum. Thus, there is a need for more accurate data for the elemental compositions of tumours and healthy cells. For the nucleus compositions and tissue types investigated, MC-D-w,D-m is a good substitute to MC-D-n,D-m for all simulated photon energies. Whether other studied surrogates are good approximations to MC-D-n,D-m depends on the target size, target composition, composition of the surrounding tissue and photon energy.

  • 16. Fernandez-Varea, Jose M.
    et al.
    Gonzalez-Munoz, Gloria
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Galassi, Mariel E.
    Wiklund, Kristin
    Lind, Bengt K.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Limitations (and merits) of PENELOPE as a track-structure code2012In: International Journal of Radiation Biology, ISSN 0955-3002, E-ISSN 1362-3095, Vol. 88, no 1-2, p. 66-70Article in journal (Refereed)
    Abstract [en]

    Purpose: To outline the limitations of PENELOPE (acronym of PENetration and Energy LOss of Positrons and Electrons) as a track-structure code, and to comment on modifications that enable its fruitful use in certain microdosimetry and nanodosimetry applications. Methods: Attention is paid to the way in which inelastic collisions of electrons are modelled and to the ensuing implications for microdosimetry analysis. Results: Inelastic mean free paths and collision stopping powers calculated with PENELOPE and two well-known optical-data models are compared. An ad hoc modification of PENELOPE is summarized where ionization and excitation of liquid water by electron impact is simulated using tables of realistic differential and total cross sections. Conclusions: PENELOPE can be employed advantageously in some track-structure applications provided that the default model for inelastic interactions of electrons is replaced by suitable tables of differential and total cross sections.

  • 17. Georg, Dietmar
    et al.
    Nyholm, Tufve
    Olofsson, Jörgen
    Kjær-Kristoffersen, Flemming
    Schnekenburger, Bruno
    Winkler, Peter
    Nyström, Håkan
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Oncology.
    Karlsson, Mikael
    Clinical evaluation of monitor unit software and the application of action levels2007In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 85, no 2, p. 306-315Article in journal (Refereed)
    Abstract [en]

    PURPOSE: The aim of this study was the clinical evaluation of an independent dose and monitor unit verification (MUV) software which is based on sophisticated semi-analytical modelling. The software was developed within the framework of an ESTRO project. Finally, consistent handling of dose calculation deviations applying individual action levels is discussed.

    MATERIALS AND METHODS: A Matlab-based software (&quot;MUV&quot;) was distributed to five well-established treatment centres in Europe (Vienna, Graz, Basel, Copenhagen, and Umeå) and evaluated as a quality assurance (QA) tool in clinical routine. Results were acquired for 226 individual treatment plans including a total of 815 radiation fields. About 150 beam verification measurements were performed for a portion of the individual treatment plans, mainly with time variable fluence patterns. The deviations between dose calculations performed with a treatment planning system (TPS) and the MUV software were scored with respect to treatment area, treatment technique, geometrical depth, radiological depth, etc.

    RESULTS: In general good agreement was found between calculations performed with the different TPSs and MUV, with a mean deviation per field of 0.2+/-3.5% (1 SD) and mean deviations of 0.2+/-2.2% for composite treatment plans. For pelvic treatments less than 10% of all fields showed deviations larger than 3%. In general, when using the radiological depth for verification calculations the results and the spread in the results improved significantly, especially for head-and-neck and for thorax treatments. For IMRT head-and-neck beams, mean deviations between MUV and the local TPS were -1.0+/-7.3% for dynamic, and -1.3+/-3.2% for step-and-shoot IMRT delivery. For dynamic IMRT beams in the pelvis good agreement was obtained between MUV and the local TPS (mean: -1.6+/-1.5%). Treatment site and treatment technique dependent action levels between +/-3% and +/-5% seem to be clinically realistic if a radiological depth correction is performed, even for dynamic wedges and IMRT.

    CONCLUSION: The software MUV is well suited for patient specific treatment plan QA applications and can handle all currently available treatment techniques that can be applied with standard linear accelerators. The highly sophisticated dose calculation model implemented in MUV allows investigation of systematic TPS deviations by performing calculations in homogeneous conditions

  • 18. Georg, Dietmar
    et al.
    Stock, Markus
    Kroupa, Bernhard
    Olofsson, Jörgen
    Nyholm, Tufve
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Oncology.
    Karlsson, Mikael
    Patient-specific IMRT verification using independent fluence-based dose calculation software: experimental benchmarking and initial clinical experience2007In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 52, no 16, p. 4981-4992Article in journal (Refereed)
    Abstract [en]

    Experimental methods are commonly used for patient-specific intensity-modulated radiotherapy (IMRT) verification. The purpose of this study was to investigate the accuracy and performance of independent dose calculation software (denoted as 'MUV' (monitor unit verification)) for patient-specific quality assurance (QA). 52 patients receiving step-and-shoot IMRT were considered. IMRT plans were recalculated by the treatment planning systems (TPS) in a dedicated QA phantom, in which an experimental 1D and 2D verification (0.3 cm3 ionization chamber; films) was performed. Additionally, an independent dose calculation was performed. The fluence-based algorithm of MUV accounts for collimator transmission, rounded leaf ends, tongue-and-groove effect, backscatter to the monitor chamber and scatter from the flattening filter. The dose calculation utilizes a pencil beam model based on a beam quality index. DICOM RT files from patient plans, exported from the TPS, were directly used as patient-specific input data in MUV. For composite IMRT plans, average deviations in the high dose region between ionization chamber measurements and point dose calculations performed with the TPS and MUV were 1.6 ± 1.2% and 0.5 ± 1.1% (1 S.D.). The dose deviations between MUV and TPS slightly depended on the distance from the isocentre position. For individual intensity-modulated beams (total 367), an average deviation of 1.1 ± 2.9% was determined between calculations performed with the TPS and with MUV, with maximum deviations up to 14%. However, absolute dose deviations were mostly less than 3 cGy. Based on the current results, we aim to apply a confidence limit of 3% (with respect to the prescribed dose) or 6 cGy for routine IMRT verification. For off-axis points at distances larger than 5 cm and for low dose regions, we consider 5% dose deviation or 10 cGy acceptable. The time needed for an independent calculation compares very favourably with the net time for an experimental approach. The physical effects modelled in the dose calculation software MUV allow accurate dose calculations in individual verification points. Independent calculations may be used to replace experimental dose verification once the IMRT programme is mature.

  • 19.
    Grönlund, Eric
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Johansson, Silvia
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Experimental and Clinical Oncology.
    Montelius, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Dose painting by numbers based on retrospectively determined recurrence probabilities2017In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 122, no 2, p. 236-241Article in journal (Refereed)
    Abstract [en]

    Background and purpose: The aim of this study is to derive "dose painting by numbers" prescriptions from retrospectively observed recurrence volumes in a patient group treated with conventional radiotherapy for head and neck squamous cell carcinoma. Materials and methods: The spatial relation between retrospectively observed recurrence volumes and pre-treatment standardized uptake values (SUV) from fluorodeoxyglucose positron emission tomography (FDG-PET) imaging was determined. Based on this information we derived SUV driven dose-response functions and used these to optimize ideal dose redistributions under the constraint of equal average dose to the tumor volumes as for a conventional treatment. The response functions were also implemented into a treatment planning system for realistic dose optimization. Results: The calculated tumor control probabilities (TCP) increased between 0.1-14.6% by the ideal dose redistributions for all included patients, where patients with larger and more heterogeneous tumors got greater increases than smaller and more homogeneous tumors. Conclusions: Dose painting prescriptions can be derived from retrospectively observed recurrence volumes spatial relation to pre-treatment FDG-PET image data. The ideal dose redistributions could significantly increase the TCP for patients with large tumor volumes and large spread in SUV from FDG-PET. The results yield a basis for prospective studies to determine the clinical value for dose painting of head and neck squamous cell carcinomas.

  • 20.
    Grönlund, Eric
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Johansson, Silvia
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Experimental and Clinical Oncology. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Sciences.
    Nyholm, Tufve
    Umea Univ, Dept Radiat Sci, Umea, Sweden.
    Thellenberg, Camilla
    Umea Univ, Dept Radiat Sci, Umea, Sweden.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Dose painting of prostate cancer based on Gleason score correlations with apparent diffusion coefficients2018In: Acta Oncologica, ISSN 0284-186X, E-ISSN 1651-226X, Vol. 57, no 5, p. 574-581Article in journal (Refereed)
    Abstract [en]

    Background: Gleason scores for prostate cancer correlates with an increased recurrence risk after radiotherapy (RT). Furthermore, higher Gleason scores correlates with decreasing apparent diffusion coefficient (ADC) data from diffusion weighted MRI (DWI-MRI). Based on these observations, we present a formalism for dose painting prescriptions of prostate volumes based on ADC images mapped to Gleason score driven dose-responses.Methods: The Gleason score driven dose-responses were derived from a learning data set consisting of pre-RT biopsy data and post-RT outcomes for 122 patients treated with a homogeneous dose to the prostate. For a test data set of 18 prostate cancer patients with pre-RT ADC images, we mapped the ADC data to the Gleason driven dose-responses by using probability distributions constructed from published Gleason score correlations with ADC data. We used the Gleason driven dose-responses to optimize dose painting prescriptions that maximize the tumor control probability (TCP) with equal average dose as for the learning sets homogeneous treatment dose.Results: The dose painting prescriptions increased the estimated TCP compared to the homogeneous dose by 0-51% for the learning set and by 4-30% for the test set. The potential for individual TCP gains with dose painting correlated with increasing Gleason score spread and larger prostate volumes. The TCP gains were also found to be larger for patients with a low expected TCP for the homogeneous dose prescription.Conclusions: We have from retrospective treatment data demonstrated a formalism that yield ADC driven dose painting prescriptions for prostate volumes that potentially can yield significant TCP increases without increasing dose burdens as compared to a homogeneous treatment dose. This motivates further development of the approach to consider more accurate ADC to Gleason mappings, issues with delivery robustness of heterogeneous dose distributions, and patient selection criteria for design of clinical trials.

  • 21.
    Karlsson, Mikael
    et al.
    Umeå Universitet.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Georg, Dietmar
    Vienna University Hospital.
    Nyholm, Tufve
    Umeå Universitet.
    Independent dose calculations, concepts and models2010Book (Refereed)
  • 22.
    Källman, Hans-Erik
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. Clin Res Ctr, SE-79182 Falun, Sweden..
    Holmberg, Rickard
    Raysearch Labs AB, Box 3297, SE-10365 Stockholm, Sweden..
    Andersson, Jonas
    Umea Univ, Dept Radiat Sci Radiat Phys, SE-90185 Umea, Sweden..
    Kull, Love
    Sunderby Hosp, Norrbotten Cty Council, Dept Med Radiat Phys, SE-97180 Lulea, Sweden..
    Tranéus, Erik
    Raysearch Labs AB, Box 3297, SE-10365 Stockholm, Sweden..
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Source modeling for Monte Carlo dose calculation of CT examinations with a radiotherapy treatment planning system2016In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 11, p. 6118-6128Article in journal (Refereed)
    Abstract [en]

    Purpose: Radiation dose to patients undergoing examinations with Multislice Computed Tomography (MSCT) as well as Cone Beam Computed Tomography (CBCT) is a matter of concern. Risk management could benefit from efficient replace rational dose calculation tools. The paper aims to verify MSCT dose calculations using a Treatment Planning System (TPS) for radiotherapy and to evaluate four different variations of bow-tie filter characterizations for the beam model used in the dose calculations. Methods: A TPS (RayStation (TM), RaySearch Laboratories, Stockholm, Sweden) was configured to calculate dose from a MSCT (GE Healthcare, Wauwatosa, WI, USA). The x-ray beam was characterized in a stationary position the by measurements of the Half-Value Layer (HVL) in aluminum and kerma along the principal axes of the isocenter plane perpendicular to the beam. A Monte Carlo source model for the dose calculation was applied with four different variations on the beam-shaping bow-tie filter, taking into account the different degrees of HVL information but reconstructing the measured kerma distribution after the bow-tie filter by adjusting the photon sampling function. The resulting dose calculations were verified by comparison with measurements in solid water as well as in an anthropomorphic phantom. Results: The calculated depth dose in solid water as well as the relative dose profiles was in agreement with the corresponding measured values. Doses calculated in the anthropomorphic phantom in the range 26-55 mGy agreed with the corresponding thermo luminescence dosimeter (TLD) measurements. Deviations between measurements and calculations were of the order of the measurement uncertainties. There was no significant difference between the different variations on the bow-tie filter modeling. Conclusions: Under the assumption that the calculated kerma after the bow-tie filter replicates the measured kerma, the central specification of the HVL of the x-ray beam together with the kerma distribution can be used to characterize the beam. Thus, within the limits of the study, a flat bow-tie filter with an HVL specified by the vendor suffices to calculate the dose distribution. The TPS could be successfully configured to replicate the beam movement and intensity modulation of a spiral scan with dose modulation, on the basis of the specifications available in the metadata of the digital images and the log file of the CT.

  • 23.
    Källman, Hans-Erik
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Medicinska och farmaceutiska vetenskapsområdet, centrumbildningar mm, Center for Clinical Research Dalarna. Falu Lasarett, Bild & Funkt Med, SE-79182 Falun, Sweden.
    Traneus, Erik
    Raysearch Labs AB, Box 3297, SE-10365 Stockholm, Sweden.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Toward automated and personalized organ dose determination in CT examinations: A comparison of two tissue characterization models for Monte Carlo organ dose calculation with a Therapy Planning System2019In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 46, no 2, p. 1012-1023Article in journal (Refereed)
    Abstract [en]

    Purpose: Computed tomography (CT) is a versatile tool in diagnostic radiology with rapidly increasing number of examinations per year globally. Routine adaption of the exposure level for patient anatomy and examination protocol cause the patients' exposures to become diversified and harder to predict by simple methods. To facilitate individualized organ dose estimates, we explore the possibility to automate organ dose calculations using a radiotherapy treatment planning system (TPS). In particular, the mapping of CT number to elemental composition for Monte Carlo (MC) dose calculations is investigated.

    Methods: Organ dose calculations were done for a female thorax examination test case with a TPS (Raystation, Raysearch Laboratories AB, Stockholm, Sweden) utilizing a MC dose engine with a CT source model presented in a previous study. The TPS's inherent tissue characterization model for mapping of CT number to elemental composition of the tissues was calibrated using a phantom with known elemental compositions and validated through comparison of MC calculated dose with dose measured with Thermo Luminescence Dosimeters (TLD) in an anthropomorphic phantom. Given the segmentation tools of the TPS, organ segmentation strategies suitable for automation were analyzed for high contrast organs, utilizing CT number thresholding and model-based segmentation, and for low contrast organs utilizing water replacements in larger tissue volumes. Organ doses calculated with a selection of organ segmentation methods in combination with mapping of CT numbers to elemental composition (RT model), normally used in radiotherapy, were compared to a tissue characterization model with organ segmentation and elemental compositions defined by replacement materials [International Commission on Radiological Protection (ICRP) model], frequently favored in imaging dosimetry.

    Results: The results of the validation with the anthropomorphic phantom yielded mean deviations from the dose to water calculated with the RT and ICRP model as measured with TLD of 1.1% and 1.5% with maximum deviations of 6.1% and 8.7% respectively over all locations in the phantom. A strategy for automated organ segmentation was evaluated for two different risk organ groups, that is, low contrast soft organs and high contrast organs. The relative deviation between organ doses calculated with the RT model and with the ICRP model varied between 0% and 20% for the thorax/upper abdomen risk organs.

    Conclusions: After calibration, the RT model in the TPS provides accurate MC dose results as compared to measurements with TLD and the ICRP model. Dosimetric feasible segmentation of the risk organs for a female thorax demonstrates a possibility for automation using the segmentation tool available in a TPS for high contrast organs. Low contrast soft organs can be represented by water volumes, but organ dose to the esophagus and thyroid must be determined using standardized organ shapes. The uncertainties of the organ doses are small compared to the overall uncertainty, at least an order of magnitude larger, in the estimates of lifetime attributable risk (LAR) based on organ doses. Large-scale and automated individual organ dose calculations could provide an improvement in cancer incidence estimates from epidemiological studies.

  • 24. Nyholm, T.
    et al.
    Olofsson, J.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    Karlsson, M.
    Modelling lateral beam quality variations in pencil kernel based photon dose calculations2006In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 16, p. 4111-4118Article in journal (Refereed)
    Abstract [en]

    Standard treatment machines for external radiotherapy are designed to yield flat dose distributions at a representative treatment depth. The common method to reach this goal is to use a flattening filter to decrease the fluence in the centre of the beam. A side effect of this filtering is that the average energy of the beam is generally lower at a distance from the central axis, a phenomenon commonly referred to as off-axis softening. The off-axis softening results in a relative change in beam quality that is almost independent of machine brand and model. Central axis dose calculations using pencil beam kernels show no drastic loss in accuracy when the off-axis beam quality variations are neglected. However, for dose calculated at off-axis positions the effect should be considered, otherwise errors of several per cent can be introduced. This work proposes a method to explicitly include the effect of off-axis softening in pencil kernel based photon dose calculations for arbitrary positions in a radiation field. Variations of pencil kernel values are modelled through a generic relation between half value layer (HVL) thickness and off-axis position for standard treatment machines. The pencil kernel integration for dose calculation is performed through sampling of energy fluence and beam quality in sectors of concentric circles around the calculation point. The method is fully based on generic data and therefore does not require any specific measurements for characterization of the off-axis softening effect, provided that the machine performance is in agreement with the assumed HVL variations. The model is verified versus profile measurements at different depths and through a model self-consistency check, using the dose calculation model to estimate HVL values at off-axis positions. A comparison between calculated and measured profiles at different depths showed a maximum relative error of 4% without explicit modelling of off-axis softening. The maximum relative error was reduced to 1% when the off-axis softening was accounted for in the calculations.

  • 25. Nyholm, Tufve
    et al.
    Olofsson, Jörgen
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    Georg, Dietmar
    Karlsson, Mikael
    Pencil kernel correction and residual error estimation for quality-index-based dose calculations2006In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 51, no 23, p. 6245-6262Article in journal (Refereed)
    Abstract [en]

    Experimental data from 593 photon beams were used to quantify the errors in dose calculations using a previously published pencil kernel model. A correction of the kernel was derived in order to remove the observed systematic errors. The remaining residual error for individual beams was modelled through uncertainty associated with the kernel model. The methods were tested against an independent set of measurements. No significant systematic error was observed in the calculations using the derived correction of the kernel and the remaining random errors were found to be adequately predicted by the proposed method.

  • 26. Nyholm, Tufve
    et al.
    Olofsson, Jörgen
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Section of Medical Physics.
    Karlsson, Mikael
    Photon pencil kernel parameterisation based on beam quality index2006In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 78, no 3, p. 347-351Article in journal (Refereed)
    Abstract [en]

    Background and purpose

    New treatment techniques in radiotherapy employ increasing dose calculation complexity in treatment planning. For an adequate check of the results coming from a modern treatment planning system, clinical tools with almost the same degree of generality and accuracy as the planning system itself are needed. To fulfil this need we propose a photon pencil kernel parameterization based on a minimum of input data that can be used for phantom scatter calculations. Through scatter integration the pencil kernel model can calculate common parameters, such as TPR or phantom scatter factors, used in various dosimetric QA (quality assurance) procedures.

    Material and methods

    The proposed model originates from an already published radially parameterized pencil kernel. A depth parameterization of the pencil kernel parameters has been introduced, based on a large database containing commissioned beam data for a commercial treatment planning system. The entire pencil kernel model demands only one photon beam quality index, TPR20,10, as input.

    Results

    By comparing the dose calculation results to the extensive experimental data set in the database, it has been possible to make a thorough analysis of the resulting accuracy. The errors in calculated doses, normalized to the reference geometry, are in most cases smaller than 2%.

    Conclusions

    The investigation shows that a pencil kernel model based only on TPR20,10 can be used for dosimetric verification purposes in megavoltage photon beams at depths below the range of contaminating electrons.

  • 27. Olofsson, Jörgen
    et al.
    Nyholm, Tufve
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    Karlsson, Mikael
    Dose uncertainties in photon pencil kernel calculations at off-axis positions2006In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 9, p. 3418-3425Article in journal (Refereed)
    Abstract [en]

    The purpose of this study was to investigate the specific problems associated with photon dose calculations in points located at a distance from the central beam axis. These problems are related to laterally inhomogeneous energy fluence distributions and spectral variations causing a lateral shift in the beam quality, commonly referred to as off-axis softening (OAS). We have examined how the dose calculation accuracy is affected when enabling and disabling explicit modeling of these two effects. The calculations were performed using a pencil kernel dose calculation algorithm that facilitates modeling of OAS through laterally varying kernel properties. Together with a multisource model that provides the lateral energy fluence distribution this generates the total dose output, i.e., the dose per monitor unit, at an arbitrary point of interest. The dose calculation accuracy was evaluated through comparisons with 264 measured output factors acquired at 5, 10, and 20 cm depth in four different megavoltage photon beams. The measurements were performed up to 18 cm from the central beam axis, inside square fields of varying size and position. The results show that calculations including explicit modeling of OAS were considerably more accurate, up to 4%, than those ignoring the lateral beam quality shift. The deviations caused by simplified head scatter modeling were smaller, but near the field edges additional errors close to 1% occurred. When enabling full physics modeling in the dose calculations the deviations display a mean value of -0.1%, a standard deviation of 0.7%, and a maximum deviation of -2.2%. Finally, the results were analyzed in order to quantify and model the inherent uncertainties that are present when leaving the central beam axis. The off-axis uncertainty component showed to increase with both off-axis distance and depth, reaching 1% (1 standard deviation) at 20 cm depth.

  • 28. Olofsson, Jörgen
    et al.
    Nyholm, Tufve
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology, Avdelningen för sjukhusfysik.
    Karlsson, Mikael
    Optimization of photon beam flatness for radiation therapy2007In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 52, no 6, p. 1735-1746Article in journal (Refereed)
    Abstract [en]

    In this work, we investigate the relation between lateral fluence/dose distributions and photon beam uniformity, possibly identifying ways to improve these characteristics. The calculations included treatment head scatter properties associated with three common types of linear accelerators in order to study their impact on the results. For 6 and 18 MV photon beams the lateral fluence distributions were optimized with respect to the resulting calculated flatness, as defined by the International Electrotechnical Commission (IEC), at 10 cm depth in six different field sizes. The limits proposed by IEC for maximum dose ratios ('horns') at the depth of dose maximum have also been accounted for in the optimization procedure. The conclusion was that typical head scatter variations among different types of linear accelerators have a very limited effect on the optimized results, which implies that the existing differences in measured off-axis dose distributions are related to non-equivalent optimization objectives. Finally, a comparison between the theoretically optimized lateral dose distributions and corresponding dose measurements for the three investigated accelerator types was performed. Although the measured data generally fall within the IEC requirements the optimized distributions show better results overall for the evaluated uniformity parameters, indicating that there is room for improved flatness performance in clinical photon beams.

  • 29. Olofsson, Jörgen
    et al.
    Nyholm, Tufve
    Georg, Dietmar
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology.
    Karlsson, Mikael
    Evaluation of uncertainty predictions and dose output for model-based dose calculations for megavoltage photon beams2006In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 33, no 7, p. 2548-2556Article in journal (Refereed)
    Abstract [en]

    In many radiotherapy clinics an independent verification of the number of monitor units (MU) used to deliver the prescribed dose to the target volume is performed prior to the treatment start. Traditionally this has been done by using methods mainly based on empirical factors which, at least to some extent, try to separate the influence from input parameters such as field size, depth, distance, etc. The growing complexity of modern treatment techniques does however make this approach increasingly difficult, both in terms of practical application and in terms of the reliability of the results. In the present work the performance of a model-based approach, describing the influence from different input parameters through actual modeling of the physical effects, has been investigated in detail. The investigated model is based on two components related to megavoltage photon beams; one describing the exiting energy fluence per delivered MU, and a second component describing the dose deposition through a pencil kernel algorithm solely based on a measured beam quality index. Together with the output calculations, the basis of a method aiming to predict the inherent calculation uncertainties in individual treatment setups has been developed. This has all emerged from the intention of creating a clinical dose/MU verification tool that requires an absolute minimum of commissioned input data. This evaluation was focused on irregular field shapes and performed through comparison with output factors measured at 5, 10, and 20 cm depth in ten multileaf collimated fields on four different linear accelerators with varying multileaf collimator designs. The measurements were performed both in air and in water and the results of the two components of the model were evaluated separately and combined. When compared with the corresponding measurements the resulting deviations in the calculated output factors were in most cases smaller than 1% and in all cases smaller than 1.7%. The distribution describing the calculation errors in the total dose output has a mean value of -0.04% and a standard deviation of 0.47%. In the dose calculations a previously developed correction of the pencil kernel was applied that managed to contract the error distribution considerably. A detailed analysis of the predicted uncertainties versus the observed deviations suggests that the predictions indeed can be used as a basis for creating action levels and tracking dose calculation errors in homogeneous media.

  • 30. Pater, P.
    et al.
    Backstrom, G.
    Enger, S.
    Villegas, Fernanda Navarro
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Seuntjens, J.
    El Naqa, I.
    On the Value of LET as a Radiation Quality Descriptor for RBE2015In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 42, no 6, p. 3469-3469Article in journal (Other academic)
  • 31. Pater, P.
    et al.
    Backstrom, G.
    Enger, S.
    Villegas, F.Fernanda Navarro
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Seuntjens, J.
    El Naga, I.
    Influence of Proton Track-Cell Nucleus Incidence Angle On Relative Biological Effectiveness2015In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 42, no 6, p. 3448-3448Article in journal (Other academic)
  • 32.
    Pater, Piotr
    et al.
    McGill Univ, Med Phys Unit, Montreal, PQ H4A 3J1, Canada..
    Backstom, Gloria
    McGill Univ, Med Phys Unit, Montreal, PQ H4A 3J1, Canada..
    Villegas, Fernanda
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Enger, Shirin A.
    McGill Univ, Med Phys Unit, Montreal, PQ H4A 3J1, Canada..
    Seuntjens, Jan
    McGill Univ, Med Phys Unit, Montreal, PQ H4A 3J1, Canada..
    El Naqa, Issam
    McGill Univ, Med Phys Unit, Montreal, PQ H4A 3J1, Canada..
    Proton and light ion RBE for the induction of direct DNA double strand breaks2016In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 5Article in journal (Refereed)
    Abstract [en]

    Purpose: To present and characterize a Monte Carlo (MC) tool for the simulation of the relative biological effectiveness for the induction of direct DNA double strand breaks (RBEDSBdirect) for protons and light ions. Methods: The MC tool uses a pregenerated event-by-event tracks library of protons and light ions that are overlaid on a cell nucleus model. The cell nucleus model is a cylindrical arrangement of nucleosome structures consisting of 198 DNA base pairs. An algorithm relying on k-dimensional trees and cylindrical symmetries is used to search coincidences of energy deposition sites with volumes corresponding to the sugar-phosphate backbone of the DNA molecule. Strand breaks (SBs) are scored when energy higher than a threshold is reached in these volumes. Based on the number of affected strands, they are categorized into either single strand break (SSB) or double strand break (DSB) lesions. The number of SBs composing each lesion (i.e., its size) is also recorded. RBEDSBdirect is obtained by taking the ratio of DSB yields of a given radiation field to a Co-60 field. The MC tool was used to obtain SSB yields, DSB yields, and RBEDSBdirect as a function of linear energy transfer (LET) for protons (H-1(+)), He-4(2+), Li-7(3+), and C-12(6+) ions. Results: For protons, the SSB yields decreased and the DSB yields increased with LET. At approximate to 24.5 keV mu m(-1), protons generated 15% more DSBs than 12C6+ ions. The RBEDSBdirect varied between 1.24 and 1.77 for proton fields between 8.5 and 30.2 keV mu m(-1), and it was higher for iso-LET ions with lowest atomic number. The SSB and DSB lesion sizes showed significant differences for all radiation fields. Generally, the yields of SSB lesions of sizes >= 2 and the yields of DSB lesions of sizes >= 3 increased with LET and increased for iso-LET ions of lower atomic number. On the other hand, the ratios of SSB to DSB lesions of sizes 2-4 did not show variability with LET nor projectile atomic number, suggesting that these metrics are independent of the radiation quality. Finally, a variance of up to 8% in the DSB yields was observed as a function of the particle incidence angle on the cell nucleus. This simulation effect is due to the preferential alignment of ion tracks with the DNA nucleosomes at specific angles. Conclusions: The MC tool can predict SSB and DSB yields for light ions of various LET and estimate RBEDSBdirect. In addition, it can calculate the frequencies of different DNA lesion sizes, which is of interest in the context of biologically relevant absolute dosimetry of particle beams.

  • 33.
    Poole, Christopher M.
    et al.
    Royal Brisbane & Womens Hosp, Canc Care Serv, Brisbane, Qld, Australia.;Univ Melbourne, Royal Hosp Women, Dept Obstet & Gynaecol, Melbourne, Vic, Australia..
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Enger, Shirin A.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Determination of subcellular compartment sizes for estimating dose variations in radiotherapy2015In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 166, no 1-4, p. 361-364Article in journal (Refereed)
    Abstract [en]

    The variation in specific energy absorbed to different cell compartments caused by variations in size and chemical composition is poorly investigated in radiotherapy. The aim of this study was to develop an algorithm to derive cell and cell nuclei size distributions from 2D histology samples, and build 3D cellular geometries to provide Monte Carlo (MC)-based dose calculation engines with a morphologically relevant input geometry. Stained and unstained regions of the histology samples are segmented using a Gaussian mixture model, and individual cell nuclei are identified via thresholding. Delaunay triangulation is applied to determine the distribution of distances between the centroids of nearest neighbour cells. A pouring simulation is used to build a 3D virtual tissue sample, with cell radii randomised according to the cell size distribution determined from the histology samples. A slice with the same thickness as the histology sample is cut through the 3D data and characterised in the same way as the measured histology. The comparison between this virtual slice and the measured histology is used to adjust the initial cell size distribution into the pouring simulation. This iterative approach of a pouring simulation with adjustments guided by comparison is continued until an input cell size distribution is found that yields a distribution in the sliced geometry that agrees with the measured histology samples. The thus obtained morphologically realistic 3D cellular geometry can be used as input to MC-based dose calculation programs for studies of dose response due to variations in morphology and size of tumour/healthy tissue cells/nuclei, and extracellular material.

  • 34. Russell, Kellie R
    et al.
    Tedgren, Asa K Carlsson
    Ahnesjö, Anders
    Uppsala University, Medicinska vetenskapsområdet, Faculty of Medicine, Department of Oncology, Radiology and Clinical Immunology. Onkologi.
    Brachytherapy source characterization for improved dose calculations using primary and scatter dose separation.2005In: Medical Physics, ISSN 0094-2405, Vol. 32, no 9, p. 2739-52Article in journal (Refereed)
    Abstract [en]

    In brachytherapy, tissue heterogeneities, source shielding, and finite patient/phantom extensions affect both the primary and scatter dose distributions. The primary dose is, due to the short range of secondary electrons, dependent only on the distribution of material located on the ray line between the source and dose deposition site. The scatter dose depends on both the direct irradiation pattern and the distribution of material in a large volume surrounding the point of interest, i.e., a much larger volume must be included in calculations to integrate many small dose contributions. It is therefore of interest to consider different methods for the primary and the scatter dose calculation to improve calculation accuracy with limited computer resources. The algorithms in present clinical use ignore these effects causing systematic dose errors in brachytherapy treatment planning. In this work we review a primary and scatter dose separation formalism (PSS) for brachytherapy source characterization to support separate calculation of the primary and scatter dose contributions. We show how the resulting source characterization data can be used to drive more accurate dose calculations using collapsed cone superposition for scatter dose calculations. Two types of source characterization data paths are used: a direct Monte Carlo simulation in water phantoms with subsequent parameterization of the results, and an alternative data path built on processing of AAPM TG43 formatted data to provide similar parameter sets. The latter path is motivated of the large amounts of data already existing in the TG43 format. We demonstrate the PSS methods using both data paths for a clinical 192Ir source. Results are shown for two geometries: a finite but homogeneous water phantom, and a half-slab consisting of water and air. The dose distributions are compared to results from full Monte Carlo simulations and we show significant improvement in scatter dose calculations when the collapsed-cone kernel-superposition algorithm is used compared to traditional table based calculations. The PSS source characterization method uses exponential fit functions derived from one-dimensional transport theory to describe both the primary and scatter dose contributions. We present data for the PSS characterization method to different 192Ir, 137Cs, and 60Cs brachytherapy sources. We also show how TG43 formatted data can be derived from our data to serve traditional treatment planning systems, as to enable for a gradual transfer to algorithms that provides improved modeling of heterogeneities in brachytherapy treatment planning.

  • 35.
    Sjöberg, Carl
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics.
    Multi-atlas based segmentation using probabilistic label fusion with adaptive weighting of image similarity measures2013In: Computer Methods and Programs in Biomedicine, ISSN 0169-2607, E-ISSN 1872-7565, Vol. 110, no 3, p. 308-319Article in journal (Refereed)
    Abstract [en]

    Label fusion multi-atlas approaches for image segmentation can give better segmentation results than single atlas methods. We present a multi-atlas label fusion strategy based on probabilistic weighting of distance maps. Relationships between image similarities and segmentation similarities are estimated in a learning phase and used to derive fusion weights that are proportional to the probability for each atlas to improve the segmentation result. The method was tested using a leave-one-out strategy on a database of 21 pre-segmented prostate patients for different image registrations combined with different image similarity scorings. The probabilistic weighting yields results that are equal or better compared to both fusion with equal weights and results using the STAPLE algorithm. Results from the experiments demonstrate that label fusion by weighted distance maps is feasible, and that probabilistic weighted fusion improves segmentation quality more the stronger the individual atlas segmentation quality depends on the corresponding registered image similarity. The regions used for evaluation of the image similarity measures were found to be more important than the choice of similarity measure.

  • 36.
    Sjöberg, Carl
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Johansson, Silvia
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    How much will linked deformable registrations decrease the quality of multi-atlas segmentation fusions?2014In: Radiation Oncology, ISSN 1748-717X, E-ISSN 1748-717X, Vol. 9, article id 251Article in journal (Refereed)
    Abstract [en]

    Background and purpose: Multi-atlas segmentation can yield better results than single atlas segmentation, but practical applications are limited by long calculation times for deformable registration. To shorten the calculation time pre-calculated registrations of atlases could be linked via a single atlas registered in runtime to the current patient. The primary purpose of this work is to investigate and quantify segmentation quality changes introduced by such linked registrations. We also determine the optimal parameters for fusing linked multi-atlas labels using probabilistic weighted fusion. Material and methods: Computed tomography images of 10 head and neck cancer patients were used as atlases, with parotid glands, submandibular glands, the mandible and lymph node levels II-IV segmented by an experienced radiation oncologist following published consensus guidelines. The change in segmentation quality scored by Dice similarity coefficient (DSC) for linking free-form deformable registrations, modeled by B-splines, was investigated for both single-and multi-atlas label fusion by using a leave-one-out approach. Results: The median decrease of the DSC was in the range 2.8% to 8.4% compared to direct registrations for all structures while reducing the computer calculation time to that of a single deformable registration. Linking several registrations showed a DSC decrease almost linear to the number of links, suggesting that extrapolation to zero links provides an observer independent measure of the inherent precision with which the segmentation guidelines can be applied. Conclusions: Linking pre-made registrations of multiple atlases via a runtime registration of a single atlas provides a feasible method for reducing computation time in multi-atlas registration.

  • 37.
    Sjöberg, Carl
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Lundmark, Martin
    Granberg, Christoffer
    Johansson, Silvia
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Oncology.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Montelius, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Clinical evaluation of multi-atlas based segmentation of lymph node regions in head and neck and prostate cancer patients2013In: Radiation Oncology, ISSN 1748-717X, E-ISSN 1748-717X, Vol. 8, p. 229-Article in journal (Refereed)
    Abstract [en]

    Background: Semi-automated segmentation using deformable registration of selected atlas cases consisting of expert segmented patient images has been proposed to facilitate the delineation of lymph node regions for three-dimensional conformal and intensity-modulated radiotherapy planning of head and neck and prostate tumours. Our aim is to investigate if fusion of multiple atlases will lead to clinical workload reductions and more accurate segmentation proposals compared to the use of a single atlas segmentation, due to a more complete representation of the anatomical variations. Methods: Atlases for lymph node regions were constructed using 11 head and neck patients and 15 prostate patients based on published recommendations for segmentations. A commercial registration software (Velocity AI) was used to create individual segmentations through deformable registration. Ten head and neck patients, and ten prostate patients, all different from the atlas patients, were randomly chosen for the study from retrospective data. Each patient was first delineated three times, (a) manually by a radiation oncologist, (b) automatically using a single atlas segmentation proposal from a chosen atlas and (c) automatically by fusing the atlas proposals from all cases in the database using the probabilistic weighting fusion algorithm. In a subsequent step a radiation oncologist corrected the segmentation proposals achieved from step (b) and (c) without using the result from method (a) as reference. The time spent for editing the segmentations was recorded separately for each method and for each individual structure. Finally, the Dice Similarity Coefficient and the volume of the structures were used to evaluate the similarity between the structures delineated with the different methods. Results: For the single atlas method, the time reduction compared to manual segmentation was 29% and 23% for head and neck and pelvis lymph nodes, respectively, while editing the fused atlas proposal resulted in time reductions of 49% and 34%. The average volume of the fused atlas proposals was only 74% of the manual segmentation for the head and neck cases and 82% for the prostate cases due to a blurring effect from the fusion process. After editing of the proposals the resulting volume differences were no longer statistically significant, although a slight influence by the proposals could be noticed since the average edited volume was still slightly smaller than the manual segmentation, 9% and 5%, respectively. Conclusions: Segmentation based on fusion of multiple atlases reduces the time needed for delineation of lymph node regions compared to the use of a single atlas segmentation. Even though the time saving is large, the quality of the segmentation is maintained compared to manual segmentation.

  • 38.
    Tilly, David
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Fast dose algorithm for generation of dose coverage probability for robustness analysis of fractionated radiotherapy2015In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 60, no 14, p. 5439-5454Article in journal (Refereed)
    Abstract [en]

    A fast algorithm is constructed to facilitate dose calculation for a large number of randomly sampled treatment scenarios, each representing a possible realisation of a full treatment with geometric, fraction specific displacements for an arbitrary number of fractions. The algorithm is applied to construct a dose volume coverage probability map (DVCM) based on dose calculated for several hundred treatment scenarios to enable the probabilistic evaluation of a treatment plan.For each treatment scenario, the algorithm calculates the total dose by perturbing a pre-calculated dose, separately for the primary and scatter dose components, for the nominal conditions. The ratio of the scenario specific accumulated fluence, and the average fluence for an infinite number of fractions is used to perturb the pre-calculated dose. Irregularities in the accumulated fluence may cause numerical instabilities in the ratio, which is mitigated by regularisation through convolution with a dose pencil kernel.Compared to full dose calculations the algorithm demonstrates a speedup factor of ~1000. The comparisons to full calculations show a 99% gamma index (2%/2 mm) pass rate for a single highly modulated beam in a virtual water phantom subject to setup errors during five fractions. The gamma comparison shows a 100% pass rate in a moving tumour irradiated by a single beam in a lung-like virtual phantom. DVCM iso-probability lines computed with the fast algorithm, and with full dose calculation for each of the fractions, for a hypo-fractionated prostate case treated with rotational arc therapy treatment were almost indistinguishable.

  • 39.
    Tilly, David
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. Elekta Instruments AB, Stockholm, Sweden.
    van de Schoot, Agustinus J A J
    Academic Medical Center, Univesity of Amsterdam, Amsterdam, The Netherlands.
    Grusell, Erik
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Bel, Arjan
    Academic Medical Center, Univesity of Amsterdam, Amsterdam, The Netherlands.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Dose coverage calculation using a statistical shape model: applied to cervical cancer radiotherapy2017In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 62, no 10, p. 4140-4159Article in journal (Refereed)
    Abstract [en]

    A comprehensive methodology for treatment simulation and evaluation of dose coverage probabilities is presented where a population based statistical shape model (SSM) provide samples of fraction specific patient geometry deformations.The learning data consists of vector fields from deformable image registration of repeated imaging giving intra-patient deformations which are mapped to an average patient serving as a common frame of reference. The SSM is created by extracting the most dominating eigenmodes through principal component analysis of the deformations from all patients. The sampling of a deformation is thus reduced to sampling weights for enough of the most dominating eigenmodes that describe the deformations.For the cervical cancer patient datasets in this work, we found seven eigenmodes to be sufficient to capture 90% of the variance in the deformations of the, and only three eigenmodes for stability in the simulated dose coverage probabilities. The normality assumption of the eigenmode weights was tested and found relevant for the 20 most dominating eigenmodes except for the first. Individualization of the SSM is demonstrated to be improved using two deformation samples from a new patient. The probabilistic evaluation provided additional information about the trade-offs compared to the conventional single dataset treatment planning.

  • 40.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology.
    Reply to the comment on ‘Monte Carlo calculated microdosimetric spread for cell nucleus-sized targets exposed to brachytherapy 125I and 192Ir sources and 60Co cell irradiation’2016In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 61, no 13, p. 5103-5106Article in journal (Refereed)
    Abstract [en]

    A discrepancy between the Monte Carlo derived relative standard deviation sigma(rel)(z) (microdosimetric spread) and experimental data was reported by Villegas et al (2013 Phys. Med. Biol. 58 6149-62) suggesting wall effects as a plausible explanation. The comment by Lindborg et al (2015 Phys. Med. Biol. 60 8621-4) concludes that this is not a likely explanation. A thorough investigation of the Monte Carlo (MC) transport code used for track simulation revealed a critical bug. The corrected MC version yielded sigma(rel)(z) values that are now within experimental uncertainty. Other microdosimetric findings are hereby communicated.

  • 41.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Bäckström, Gloria
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Energy deposition clustering as a functional radiation quality descriptor for modelling relative biological effectiveness2016In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 43, no 12, p. 6322-6335Article in journal (Refereed)
    Abstract [en]

    Purpose: To explore the use of the frequency of the energy deposition (ED) clusters of different sizes (cluster order, CO) as a surrogate (instead of, e.g., LET) classification of the physical characteristics of ionizing radiation at a nanometer scale, to construct a framework for the calculation of relative biological effectiveness (RBE) with cell survival as endpoint.

    Methods: The frequency of cluster order f(CO) is calculated by sorting the ED sites generated with the Monte Carlo track structure code LIonTrack into clusters based on a single parameter called the cluster distance d(C) being the maximum allowed distance between two neighboring EDs belonging to a cluster. Published cell survival data parameterized with the linear-quadratic (LQ) model for V79 cells exposed to 15 different radiation qualities (including brachytherapy sources, proton, and carbon ions) were used as input to a fitting procedure, designed to determine a weighting function w(CO) that describes the capacity of a cluster of a certain CO to damage the cell's sensitive volume. The proposed framework uses both f(CO) and w(CO) to construct surrogate based functions for the LQ parameters a and beta from which RBE values can be derived.

    Results: The results demonstrate that radiation quality independent weights w(CO) exist for both the a and beta parameters. This enables the calculation of a values that correlate to their experimental counterparts within experimental uncertainties (relative residual of 15% for d(C) = 2.5 nm). The combination of both a and beta surrogate based functions, despite the higher relative residuals for beta values, yielded an RBE function that correlated to experimentally derived RBE values (relative residual of 16.5% for d(C) = 2.5 nm) for all radiation qualities included in this work.

    Conclusions: The f(CO) cluster characterization of ionizing radiation at a nanometer scale can effectively be used to calculate particle and energy dependent a and beta values to predict RBE values with potential applications to, e.g., treatment planning systems in radiotherapy. (C) 2016 American Association of Physicists in Medicine.

  • 42.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Microdosimetric spread for cell-sized targets exposed to 60Co, 192Ir and 125I sources2015In: Radiation Protection Dosimetry, ISSN 0144-8420, E-ISSN 1742-3406, Vol. 166, no 1-4, p. 365-368Article in journal (Refereed)
    Abstract [en]

    The magnitude of the spread in specific energy deposition per cell may be a confounding factor in dose–response analysis motivating derivation of explicit data for the most common brachytherapy isotopes 125I and 192Ir, and for 60Co radiation frequently used as reference in RBE studies. The aim of this study is to analyse the microdosimetric spread as given by the frequency distribution of specific energy for a range of doses imparted by 125I, 192Ir and 60Co sources. An upgraded version of the Monte Carlo code PENELOPE was used for scoring energy deposition distributions in liquid water for each of the radiation qualities. Frequency distributions of specific energy were calculated according to the formalism of Kellerer and Chmelevsky. Results indicate that the magnitude of the microdosimetric spread increases with decreasing target size and decreasing energy of the radiation quality. Within the clinical relevant dose range (1 to 100 Gy), the spread does not exceed 4 % for 60Co, 5 % for 192Ir and 6 % for 125I. The frequency distributions can be accurately approximated with symmetrical normal distributions at doses down to 0.2 Gy for 60Co, 0.1 Gy for 192Ir and 0.08 Gy for 125I.

  • 43.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Monte Carlo calculated microdosimetric spread for cell nucleus-sized targets exposed to brachytherapy 125I and 192Ir sources and 60Co cell irradiation2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 17, p. 6149-6162Article in journal (Refereed)
    Abstract [en]

    The stochastic nature of ionizing radiation interactions causes a microdosimetric spread in energy depositions for cell or cell nucleus-sized volumes. The magnitude of the spread may be a confounding factor in dose response analysis. The aim of this work is to give values for the microdosimetric spread for a range of doses imparted by 125I and 192Ir brachytherapy radionuclides, and for a 60Co source. An upgraded version of the Monte Carlo code PENELOPE was used to obtain frequency distributions of specific energy for each of these radiation qualities and for four different cell nucleus-sized volumes. The results demonstrate that the magnitude of the microdosimetric spread increases when the target size decreases or when the energy of the radiation quality is reduced. Frequency distributions calculated according to the formalism of Kellerer and Chmelevsky using full convolution of the Monte Carlo calculated single track frequency distributions confirm that at doses exceeding 0.08 Gy for 125I, 0.1 Gy for 192Ir, and 0.2 Gy for 60Co, the resulting distribution can be accurately approximated with a normal distribution. A parameterization of the width of the distribution as a function of dose and target volume of interest is presented as a convenient form for the use in response modelling or similar contexts.

  • 44.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science. Elekta Instrument AB.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Target size variation in microdosimetric distributions and its impact on the linear-quadratic parameterization of cell survival.2018In: Radiation Research, ISSN 0033-7587, E-ISSN 1938-5404, Vol. 190, p. 504-512Article in journal (Refereed)
    Abstract [en]

    The linear-quadratic (LQ) parameterization of survival fraction SF(D) inherently assumes that all cells in a population get the same dose D, albeit the distribution of specific energy z over the individual cells f(z,D) can be very wide. From these microdosimetric distributions, which are target size dependent, we estimate the size of the cellular sensitive volume by analysing its influence on the LQ parameterization of cell survival. A Monte Carlo track structure code was used to simulate detailed tracks from a 60Co source as well as proton and carbon ions of various energies. From these tracks, f(z,D) distributions were calculated for spherical targets with diameters ranging from 10 nm to 12 µm. A cell survival function based on f(z,D) was fitted to published experimental LQ α values, revealing an intrinsic limitation that target size imposes on the usage of f(z,D) to describe the linear term of the LQ parameterization. The results indicate that such threshold volume arises naturally from the relationship between the particle´s probability of no-hit and the probability of cell survival. Further analysis led to the proposal of a radiobiological property yf,MID, defined as the mean lineal energy corresponding to the target size that allows equivalence between the mean inactivation dose (MID) and the mean specific energy z1.  The fact that z1 is an increasing continuous function of target size within the range of biological targets of interest in radiobiology, ensures the uniqueness of yf,MID for any radiation quality, thus, its potential usefulness in modelling. In conclusion, an accurate estimation of such threshold volumes may be useful for improving modelling of cell survival curves.

  • 45.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Bäckström, Gloria
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences.
    Cluster pattern analysis of energy deposition sites for the brachytherapy sources 103Pd, 125I, 192Ir, 137Cs, and 60Co2014In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 59, no 18, p. 5531-5543Article in journal (Refereed)
    Abstract [en]

    Analysing the pattern of energy depositions may help elucidate differences in the severity of radiation-induced DNA strand breakage for different radiation qualities. It is often claimed that energy deposition (ED) sites from photon radiation form a uniform random pattern, but there is indication of differences in RBE values among different photon sources used in brachytherapy. The aim of this work is to analyse the spatial patterns of EDs from 103Pd, 125I, 192Ir, 137Cs sources commonly used in brachytherapy and a 60Co source as a reference radiation. The results suggest that there is both a non-uniform and a uniform random component to the frequency distribution of distances to the nearest neighbour ED. The closest neighbouring EDs show high spatial correlation for all investigated radiation qualities, whilst the uniform random component dominates for neighbours with longer distances for the three higher mean photon energy sources (192Ir, 137Cs, and 60Co). The two lower energy photon emitters (103Pd and 125I) present a very small uniform random component. The ratio of frequencies of clusters with respect to 60Co differs up to 15% for the lower energy sources and less than 2% for the higher energy sources when the maximum distance between each pair of EDs is 2 nm. At distances relevant to DNA damage, cluster patterns can be differentiated between the lower and higher energy sources. This may be part of the explanation to the reported difference in RBE values with initial DSB yields as an endpoint for these brachytherapy sources.

  • 46.
    Villegas, Fernanda
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Tilly, Nina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology. Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Medical Radiation Sciences. Elekta Instrument AB, Box7593, SE-10393 Stockholm, Sweden.
    Bäckström, Gloria
    McGill Univ, Dept Oncol, Med Phys Unit, Montreal, PQ, Canada; McGill Univ, Ctr Hlth, Res Inst, Montreal, PQ, Canada.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Corrigendum to ’Cluster pattern analysis of energy deposition sites for the brachytherapy sources 103Pd,125I,192Ir,137Cs and 60Co’, PMB 59 (2014) 5531-432016In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 61, no 15, p. 5883-5886Article in journal (Refereed)
  • 47. Wang, Ruoxi
    et al.
    Pittet, Patrick
    Ribouton, Julien
    Lu, Guo-Neng
    Chaikh, Abdulhamid
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Implementation and validation of a fluence pencil kernels model for GaN-based dosimetry in photon beam radiotherapy2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 19, p. 6701-6712Article in journal (Refereed)
    Abstract [en]

    Gallium nitride (GaN), a direct-gap semiconductor that is radioluminescent, can be used as a transducer yielding a high signal from a small detecting volume and thus potentially suitable for use in small fields and for high dose gradients. A common drawback of semiconductor dosimeters with effective atomic numbers higher than soft tissues is that their responses depend on the presence of low energy photons for which the photoelectric cross section varies strongly with atomic number, which may affect the accuracy of dosimetric measurements. To tackle this 'over-response' issue, we propose a model for GaN-based dosimetry with readout correction. The local photon spectrum is calculated by convolving fluence pencil kernel spectra with the beam aperture fluence distribution. The response of a GaN detector is modelled by combining large cavity theory and small cavity theory for the low and high energy components of the local spectrum. Monte Carlo simulations are employed for determination of specific correction factors for different GaN transducer sizes and irradiation conditions. Some model parameters such as the cut-off energy and partitioning energy are discussed. The accuracy of the GaN dosimetric response model has been evaluated for tissue phantom ratio experiments along the central axis. These experiments have shown that calculated and measured GaN responses stay within +/- 3% at all depths beyond the build-up depth. The calculated GaN response factor is also in good agreement with measured data (+/- 2.5%). The validated model with response compensation improves significantly the accuracy of dosimetric measurements: below 2.5% deviation as compared to 13% without compensation, for a 10 x 10 cm(2) field, at depth from 1.5 to 22 cm.

  • 48. Wang, Ruoxi
    et al.
    Pittet, Patrick
    Ribouton, Julien
    Lu, Guo-Neng
    Galvan, Jean-Marc
    Jalade, Patrice
    Balosso, Jacques
    Ahnesjo, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Bi-Crystal compensation method for the over-response of solid-state dosimetry2014In: Materials and Applications for Sensors and Transducers III, 2014, Vol. 605, p. 540-543Conference paper (Refereed)
    Abstract [en]

    Solid-state dosimetry employs highly sensitive semiconductors such as Gallium Nitride (GaN) and Silicon (Si), but they have a common drawback of over response compared to tissues for low-energy scattered photons, which induces inacceptable errors for radiotherapy application. To tackle this issue, we propose a compensation method consisting in using two different materials of dosimetric interest with different atomic numbers. Their responses are denoted as SC1 and SC2. The response ratio SC1/water as a function of the ratio SC1/SC2 exhibits a monotonic curve that can serve as reference to compensate the over-response of SC1. To validate this method, we have studied the dosimetric response of GaN (0.1 mm(3)) and Si crystals (2.5 mm(3)) by simulations, using a validated model based on the general cavity theory in a homogeneous water phantom. The dosimetric response of GaN and Si calculated using the model has errors within 2.5% compared to measured data. The local fluence spectra have been obtained by convolution of pencil beam kernel built by Monte Carlo simulations for different clinical irradiation conditions with field size (from 5x5 cm(2) up to 20x20 cm(2)) at depth in the phantom (from 2 cm to 25 cm). The obtained results confirm a monotone relationship between GaN/water dose ratio and GaN/Si dose ratio. The reference curve is independent of irradiation conditions (field size, dosimeter position...), and allows determination of compensation value by identification.

  • 49.
    Wikström, Kenneth
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Isacsson, Ulf
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Nilsson, Kristina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Experimental and Clinical Oncology.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Immunology, Genetics and Pathology, Medical Radiation Science.
    Reproducibility of heart and thoracic wall positionin repeated deep inspiration breath holds forradiotherapy of left-sided breast cancer patients2018In: Acta Oncologica, ISSN 0284-186X, E-ISSN 1651-226X, Vol. 57, no 10, p. 1318-1324Article in journal (Refereed)
    Abstract [en]

    Background: Deep inspiration breath hold (DIBH) for radiotherapy of left-sided breast cancer patientscan effectively move the heart away from the target and reduce the heart dose compared to treatmentsin free breathing. This study aims to investigate the positional reproducibility of heart edge(HE) and thoracic wall (TW) during repeated DIBHs.

    Material and methods: At three occasions, 11 left-sided breast cancer patients were CT imaged during6 minutes of repeated DIBHs with 60 cine CT series. The series were evenly distributed over threebed positions and for each bed position, the heart edge associated maximum heart distance (MHD)and thoracic wall-associated maximum lung distance (MLD) from a reference line were retrospectivelyanalyzed. The high temporal resolution of the CT series enabled intrinsic heart movements to beresolved from breath hold variations. A body surface laser scanning system continuously extracted thethorax height and displayed it in a pair of goggles for patient feedback. To check for ‘fake-breathing’movements, e.g. that the patient lifts its back from the couch to reach DIBH, the couch-to-spine distancewas also measured in all CT series.

    Results: The analysis was done for 1432 cine CTs captured during 292 breath holds. The DIBH movedthe heart on average 15mm in medial direction compared with free breathing. For the three bed positionsstudied, the mean value of the max range, across all patients, was between 11–13mm for theMHD and 4–8mm for the MLD. The MHD variation due to breath hold variation was twice as large asthe MHD variation due to intrinsic heart movement. The couch-to-spine distance varied less than3mm for all fractions, i.e., no fake-breathing was discovered.

    Conclusions: The heart edge and thoracic wall reproducibility was high in relation to the medial heartdisplacement induced by the DIBH.

  • 50.
    Wikström, Kenneth
    et al.
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Nilsson, Kristina
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Isacsson, Ulf
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    Ahnesjö, Anders
    Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science.
    A comparison of patient position displacements from body surface laser scanning and cone beam CT bone registrations for radiotherapy of pelvic targets2014In: Acta Oncologica, ISSN 0284-186X, E-ISSN 1651-226X, Vol. 53, no 2, p. 268-277Article in journal (Refereed)
    Abstract [en]

    Background and purpose

    Optical surface detection has attractive features as a mean in radiotherapy for patient positioning tasks such as set-up, monitoring and gating. To aid in hitting radiotherapy targets the correlation between detected surface displacements and internal structure displacements is crucial. In this study, we compare set-up displacements derived from a body surface laser scanning (BSLS) system to displacements derived from bone registrations with a cone beam computed tomography (CBCT) system in order to quantify the accuracy and applicability of BSLS for fractionated treatments in the pelvic region.

    Material and methods

    Displacements from concurrent BSLS and CBCT registrations were compared for 40 patients treated in the pelvic region for a total of 170 set-ups. Surface data captured by BSLS at the first treatment fraction (BSLSref) was used as main reference for the BSLS system, while bony structures from the planning CT were used as a reference for the CBCT method. As comparison, the patient outline extracted from the planning CT was used as BSLS reference (CTref). The displacements detected by the CBCT system (skin-marks-only) was also used for comparison.

    Results

    The mean differences (+/- 1 SD) between the BSLS and CBCT displacements were -0.01 (+0.17) cm, 0.00 (+0.21) cm and 0.01 (+0.17) cm in the lateral, longitudinal and vertical directions, respectively. The median length of the difference was 0.26 cm (0.24-0.29 cm, 95% CI). The median of the difference between CBCT and BSLS displacements based on CTref was 0.37 cm (0.30-0.39 cm) and the median for skin-marks-only was 0.38 cm (0.34-0.42 cm).

    Conclusions

    The BSLS system is a good supplement to the CBCT system for accurate set-up for fractions when no CBCT is deemed necessary for pelvic targets. Inter-fractional skin movement in relation to bone was estimated to be 0.2 cm in the lateral (X), longitudinal (Y) and vertical direction (Z), respectively.

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