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Fast IMRT with narrow high energy scanned photon beams
Stockholm University, Faculty of Science, Department of Physics.
Stockholm University, Faculty of Science, Department of Physics. (Medicinsk strålningsfysik)
Stockholm University, Faculty of Science, Department of Physics.
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2011 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 38, no 8, 4774-4784 p.Article in journal (Refereed) Published
Abstract [en]

Purpose: Since the first publications on intensity modulated radiation therapy (IMRT) in the early 1980s almost all efforts have been focused on fairly time consuming dynamic or segmental multileaf collimation. With narrow fast scanned photon beams, the flexibility and accuracy in beam shaping increases, not least in combination with fast penumbra trimming multileaf collimators. Previously, experiments have been performed with full range targets, generating a broad bremsstrahlung beam, in combination with multileaf collimators or material compensators. In the present publication, the first measurements with fast narrow high energy (50 MV) scanned photon beams are presented indicating an interesting performance increase even though some of the hardware used were suboptimal. Methods: Inverse therapy planning was used to calculate optimal scanning patterns to generate dose distributions with interesting properties for fast IMRT. To fully utilize the dose distributional advantages with scanned beams, it is necessary to use narrow high energy beams from a thin bremsstrahlung target and a powerful purging magnet capable of deflecting the transmitted electron beam away from the generated photons onto a dedicated electron collector. During the present measurements the scanning system, purging magnet, and electron collimator in the treatment head of the MM50 racetrack accelerator was used with 3-6 mm thick bremsstrahlung targets of beryllium. The dose distributions were measured with diodes in water and with EDR2 film in PMMA. Monte Carlo simulations with GEANT4 were used to study the influence of the electrons transmitted through the target on the photon pencil beam kernel. Results: The full width at half-maximum (FWHM) of the scanned photon beam was 34 mm measured at isocenter, below 9.5 cm of water, 1 m from the 3 mm Be bremsstrahlung target. To generate a homogeneous dose distribution in a 10 x 10 cm(2) field, the authors used a spot matrix of 100 equal intensity beam spots resulting in a uniformity of collimated 80%-20% penumbra of 9 mm at a primary electron energy of 50 MeV. For the more complex cardioid shaped dose distribution, they used 270 spots, which at a pulse repetition frequency of 200 Hz is completed every 1.36 s. Conclusions: The present measurements indicate that the use of narrow scanned photon beams is a flexible and fast method to deliver advanced intensity modulated beams. Fast scanned photon IMRT should, therefore, be a very interesting modality in the delivery of biologically optimized radiation therapy with the possibility for in vivo treatment verification with PET-CT imaging.

Place, publisher, year, edition, pages
2011. Vol. 38, no 8, 4774-4784 p.
Keyword [en]
IMRT, scanned photon beams, Monte Carlo
National Category
Radiology, Nuclear Medicine and Medical Imaging
Identifiers
URN: urn:nbn:se:su:diva-69259DOI: 10.1118/1.3615059ISI: 000293417500038OAI: oai:DiVA.org:su-69259DiVA: diva2:476489
Note
authorCount :5 Affiliation for 4 of the authors of the Stockholm University: Department of Medical Radiation Physics, Karolinska Institutet and Stockholm UniversityAvailable from: 2012-01-12 Created: 2012-01-11 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Development of improved radiation therapy techniques using narrow scanned photon beams
Open this publication in new window or tab >>Development of improved radiation therapy techniques using narrow scanned photon beams
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The present thesis is focused on the development and application of narrow scanned high energy photon beam for radiation therapy. The introduction of physically and biologically optimized intensity modulated radiation therapy (IMRT) requires a flexible and accurate dose delivery method to maximize the treatment outcome. Narrow scanned photon beams is a fast option for IMRT since it is not dependent on mechanically moving heavy collimator leafs and largely independent of the complexity of the desired dose distribution. Scanned photon beams can be produced by scanning an electron beam of low emittance, incident on a thin bremsstrahlung target of low atomic number. The large fraction of high energy electrons that are transmitted through the target has to be removed by a strong purging magnet. In the thesis a strong purging magnet, coupled with a magnetic scanning magnet, is presented for an intrinsic electron energy of 50 - 75 MeV and a source to isocenter distance of 75 cm. The available scan area at isocenter can be as large as 43 x 40 cm2 for an incident electron energy of 50 MeV and 28 x 40 cm2 at 75 MeV.

By modifying the existing treatment head of the racetrack microtron MM50, it was possible to experimentally produce relevant dose distributions with interesting properties from 50 MV scanned narrow photon beams while deflecting the transmitted electrons onto a simplified electron stopper. The deflection of the transmitted electrons was studied both experimentally and by the Monte Carlo method. With high energy photons, treatment verification is possible through PET-CT imaging of the positron annihilations induced by photonuclear reactions in the patient. Narrow scanned high energy photon beams is the ideal beam quality since the activation efficiency and the effective photon energy will be more uniform than the filtered photon beam from a full range bremsstrahlung target.

The induced 11C activity 50 MV by scanned narrow photon beams was measured using PET-CT imaging and compared with Monte Carlo simulations. The combination of fast flexible dose delivery with treatment verification using PET-CT imaging makes narrow high energy scanned photon beams a very interesting treatment modality for biologically optimized adaptive radiation therapy.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2010. 36 p.
Keyword
IMRT, Scanned photon beams, PET-CT treatment verification, Monte Carlo
National Category
Natural Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-45809 (URN)978-91-7447-189-2 (ISBN)
Public defence
2010-12-10, Föreläsningssalen, Cancer Centrum Karolinska, R8:00, Karolinska universitetssjukhuset, Solna, 14:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted.Available from: 2012-01-12 Created: 2010-11-11 Last updated: 2012-01-12Bibliographically approved
2. Verification of high energy photon therapy based on PET/CT imaging of photonuclear reactions
Open this publication in new window or tab >>Verification of high energy photon therapy based on PET/CT imaging of photonuclear reactions
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

For classical and intensity modulated radiation therapy of deep-seated tumors, high-energy photons are the optimal radiation modality from an integral dose point of view. By using narrow scanned beams the treatment outcome can be improved substantially by delivering biologically optimized intensity modulated distributions often with sharp dose gradients. This requires using photons with energies well above 15 MV enabling verification of the treatment delivery in vivo by PET/CT imaging in a manner not previously possible. This new technique is based on the production of positron emitting radionuclides when the incoming high-energy photons interact through photonuclear reactions with the body tissues. The produced radionuclides, commonly 11C, 15O and 13N can then be monitored by PET and the distribution of activated nuclei show exactly where the radiation has penetrated the patient. In the subcutaneous fat, present in all humans, a high induced activity produces a perfect visualization of the location and even the intensity modulation of the incident beams. The reason for this is the high carbon content in combination with a low biological perfusion in fat tissues. Errors in the patient positioning such as setup errors or misplacement of the beams will thus show up in the PET images as a deviation from the actual radiation treatment plan. Interestingly, the imaged activity distribution from the subcutaneous fat also visualizes how the dose delivery can be deformed when the patient is erroneously positioned on the treatment couch as seen on the cover figure. Furthermore, the different half-lives of the produced radionuclides (20 min, 2 min, and 10 min, for 11C, 15O and 13N, respectively) allows for analysis of the dynamic behavior of tissue activity with the possibility of retrieving information such as tissue composition, biological and physical half-lives. The present thesis shows that considerable clinical information regarding the treatment delivery with high-energy photon beams can be obtained using PET/CT imaging. Although the study is based on the use of 50 MV photons the method may apply for beams with energies > 20 MV at higher doses.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2012. 49 p.
Keyword
Photonuclear reactions, PET/CT treatment verification, High-energy photon therapy
National Category
Natural Sciences
Research subject
Medical Radiation Physics
Identifiers
urn:nbn:se:su:diva-72385 (URN)978-91-7447-461-9 (ISBN)
Public defence
2012-03-09, föreläsningssalen, Radiumhemmet, Karolinska universitetssjukhuset, Solna, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Submitted. Paper 3: Submitted. Paper: Manuscript.

Available from: 2012-02-16 Created: 2012-02-09 Last updated: 2017-10-11Bibliographically approved

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