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Protons, other Light Ions, and 60Co Photons: Study of Energy Deposit Clustering via Track Structure Simulations
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Radiology, Oncology and Radiation Science, Section of Medical Physics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Radiotherapy aims to sterilize cancer cells through ionization induced damages to their DNA whilst trying to reduce dose burdens to healthy tissues. This can be achieved to a certain extent by optimizing the choice of radiation to treat the patient, i.e. the types of particles and their energy based on their specific interaction patterns. In particular, the formation of complex clusters of energy deposits (EDs) increases with the linear energy transferred for a given particle. These differences cause variation in the relative biological effectiveness (RBE). The complexity of ED clusters might be related to complex forms of DNA damage, which are more difficult to repair and therefore prone to inactivate the cells. Hence, mapping of the number and complexity of ED clusters for different radiation qualities could aid to infer a surrogate measure substituting physical dose and LET as main predictors for the RBE .  

In this work the spatial patterns of EDs at the nanometre scale were characterized for various energies of proton, helium, lithium and carbon ions. A track structure Monte Carlo code, LIonTrack, was developed to accurately simulate the light ion tracks in liquid water. The methods to emulate EDs at clinical dose levels in cell nucleus-sized targets for both 60Co photons and light ions were established, and applied to liquid water targets. All EDs enclosed in such targets were analyzed with a specifically developed cluster algorithm where clustering was defined by a single parameter, the maximum distance between nearest neighbour EDs. When comparing measured RBE for different radiation qualities, there are cases for which RBE do not  increase with LET but instead increase with the frequencies of high order ED clusters.

A test surrogate-measure based on ED cluster frequencies correlated to parameters of experimentally determined cell survival. The tools developed in this thesis can facilitate future exploration of semi-mechanistic modelling of the RBE.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 55 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 930
Keyword [en]
Proton, light ion, Co-60 photon, track structure Monte Carlo code, clustering patterns of energy deposit, RBE
National Category
Medical and Health Sciences
Research subject
Medical Radiophysics
Identifiers
URN: urn:nbn:se:uu:diva-206385ISBN: 978-91-554-8736-2 (print)OAI: oai:DiVA.org:uu-206385DiVA: diva2:644513
Public defence
2014-03-28, Skoogsalen, Akademiska Sjukhuset, Ing. 78-79, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2014-03-06 Created: 2013-08-30 Last updated: 2014-04-29Bibliographically approved
List of papers
1. Monte Carlo simulation and analysis of proton energy-deposition patterns in the Bragg peak
Open this publication in new window or tab >>Monte Carlo simulation and analysis of proton energy-deposition patterns in the Bragg peak
2008 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 53, no 11, 2857-75 p.Article in journal (Refereed) Published
Abstract [en]

The spatial pattern of energy depositions is crucial for understanding the mechanisms that modify the relative biological effectiveness of different radiation qualities. In this paper, we present data on energy-deposition properties of mono-energetic protons (1-20 MeV) and their secondary electrons in liquid water. Proton-impact ionization was described by means of the Hansen-Kocbach-Stolterfoht doubly differential cross section (DDCS), thus modelling both the initial energy and angle of the emitted electron. Excitation by proton impact was included to account for the contribution of this interaction channel to the electronic stopping power of the projectile. Proton transport was implemented assuming track-segment conditions, whereas electrons were followed down to 50 eV by the Monte Carlo code PENELOPE. Electron intra-track energy-deposition properties, such as slowing-down and energy-imparted spectra of electrons, were calculated. Furthermore, the use of DDCSs enabled the scoring of electron inter-track properties. We present novel results for 1, 5 and 20 MeV single-proton-track frequencies of distances between the nearest inter- (e(-)-e(-), e(-)-H+) and intra-track (e(-)-e(-), e(-)-H+, H+-H+) energy-deposition events. By setting a threshold energy of 17.5 eV, commonly employed as a surrogate to discriminate for elementary damage in the DNA, the variation in these frequencies was studied as well. The energy deposited directly by the proton represents a large amount of the total energy deposited along the track, but when an energy threshold is adopted the relative contribution of the secondary electrons becomes larger for increasing energy of the projectile. We found that the frequencies of closest energy-deposition events per nanometre decrease with proton energy, i.e. for lower proton energies a denser ionization occurs, following the trend of the characteristic LET curves. In conclusion, considering the energy depositions due to the delta electrons and at the core of the track, 1 MeV protons have an intrinsic capability of generating about five times more dual depositions within the characteristic 2 nm of the DNA-chain structure than 20 MeV protons.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-103247 (URN)10.1088/0031-9155/53/11/007 (DOI)000256352000008 ()18460751 (PubMedID)
Available from: 2009-05-15 Created: 2009-05-15 Last updated: 2017-12-13Bibliographically approved
2. Limitations (and merits) of PENELOPE as a track-structure code
Open this publication in new window or tab >>Limitations (and merits) of PENELOPE as a track-structure code
Show others...
2012 (English)In: International Journal of Radiation Biology, ISSN 0955-3002, E-ISSN 1362-3095, Vol. 88, no 1-2, 66-70 p.Article in journal (Refereed) Published
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.

Keyword
Radiation physics, Monte Carlo simulation, microdosimetry
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-168096 (URN)10.3109/09553002.2011.598209 (DOI)000298666000012 ()
Available from: 2012-02-08 Created: 2012-02-06 Last updated: 2017-12-08Bibliographically approved
3. Track structure of protons and other light ions in liquid water: Applications of the LIonTrack code at the nanometer scale
Open this publication in new window or tab >>Track structure of protons and other light ions in liquid water: Applications of the LIonTrack code at the nanometer scale
Show others...
2013 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 40, no 6, 064101- p.Article in journal (Refereed) Published
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.

Keyword
track structure of protons and light ions, spatial patterns of energy deposits, Monte Carlo code, CDW-EIS model
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-204126 (URN)10.1118/1.4803464 (DOI)000319889100042 ()
Available from: 2013-07-22 Created: 2013-07-22 Last updated: 2017-12-06Bibliographically approved
4. Spatial energy-deposit patterns generated by light ions and 60Co photons in cell nucleus-sized targets
Open this publication in new window or tab >>Spatial energy-deposit patterns generated by light ions and 60Co photons in cell nucleus-sized targets
(English)Manuscript (preprint) (Other academic)
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-206314 (URN)
Available from: 2013-08-30 Created: 2013-08-30 Last updated: 2014-03-06

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