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Title: SU-F-T-131: No Increase in Biological Effectiveness Through Collimator Scattered Low Energy Protons

Abstract

Purpose: To reduce the lateral penumbra of low-energy proton beams, brass collimators are often used in spot-scanning proton therapy (SSPT). This study investigates the increase in biological effectiveness through collimator scattered protons in SSPT. Methods: The SSPT system of the Hokkaido University Hospital Proton Beam Therapy Center, which consists of a scanning nozzle, a 2-cm thick brass collimator, and a 4-cm thick energy absorber, was simulated with our validated Geant4 Monte Carlo code (ver. 9.3). A water phantom was irradiated with proton pencil beams of 76, 110, and 143 MeV. The tested collimator opening areas (COA) were 5×5, 10×10, and 15×15 cm{sup 2}. Comparisons were made among the dose-averaged LET values of protons that hit the collimators (LETDColl), protons that did not hit the collimators (LETDNoColl), and all protons (LETDTotal). X-ray equivalent doses (Deq) were calculated using the linear-quadratic model with LETDNoColl and LETDTotal, and their maximum difference was determined over regions where the physical dose was greater than 10% of the peak dose of 2 Gy. Results: The ratio of the dose contribution of collimator scattered protons to that of all protons, defined as λ, was large at high proton energies and large COAs. The maximum λ value rangedmore » from 3% (76 MeV, 5×5 cm{sup 2}) to 29% (143 MeV, 15×15 cm{sup 2}). Moreover, a large difference between LETDColl and LETDNoColl was only found in regions where λ was below 20% (ΔLETD > 2 keV/µm) and 8% (ΔLETD > 5 keV/µm). Consequently, the maximum difference between LETDNoColl and LETDTotal was as small as 0.8 keV/µm in all simulated voxels, and the difference of Deq reached a maximum of 1.5% that of the peak dose obtained at the water surface with a 76 MeV beam. Conclusion: Although collimator scattered protons have high LET, they only increase the physical dose, not the biological effectiveness.« less

Authors:
; ; ; ; ; ;  [1]; ;  [2];  [3]
  1. Proton Beam Therapy Center, Hokkaido University Hospital, Sapporo, Hokkaido (Japan)
  2. Department of Radiation Medicine, Graduate School of Medicine, Hokkaido University, Sapporo, Hokkaido (Japan)
  3. Division of Quantum Science and Engineering, Graduate School of Engineering, Sapporo, Hokkaido (Japan)
Publication Date:
OSTI Identifier:
22642372
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
07 ISOTOPES AND RADIATION SOURCES; 60 APPLIED LIFE SCIENCES; COLLIMATORS; DOSE EQUIVALENTS; EDUCATIONAL FACILITIES; KEV RANGE 01-10; MEV RANGE 100-1000; MEV RANGE 10-100; MONTE CARLO METHOD; PROTON BEAMS; SIMULATION; X RADIATION

Citation Formats

Matsuura, T, Takao, S, Matsuzaki, Y, Fujii, Y, Fujii, T, Umegaki, K, Shirato, H, Maeda, K, Koyano, H, and Ueda, H. SU-F-T-131: No Increase in Biological Effectiveness Through Collimator Scattered Low Energy Protons. United States: N. p., 2016. Web. doi:10.1118/1.4956267.
Matsuura, T, Takao, S, Matsuzaki, Y, Fujii, Y, Fujii, T, Umegaki, K, Shirato, H, Maeda, K, Koyano, H, & Ueda, H. SU-F-T-131: No Increase in Biological Effectiveness Through Collimator Scattered Low Energy Protons. United States. doi:10.1118/1.4956267.
Matsuura, T, Takao, S, Matsuzaki, Y, Fujii, Y, Fujii, T, Umegaki, K, Shirato, H, Maeda, K, Koyano, H, and Ueda, H. 2016. "SU-F-T-131: No Increase in Biological Effectiveness Through Collimator Scattered Low Energy Protons". United States. doi:10.1118/1.4956267.
@article{osti_22642372,
title = {SU-F-T-131: No Increase in Biological Effectiveness Through Collimator Scattered Low Energy Protons},
author = {Matsuura, T and Takao, S and Matsuzaki, Y and Fujii, Y and Fujii, T and Umegaki, K and Shirato, H and Maeda, K and Koyano, H and Ueda, H},
abstractNote = {Purpose: To reduce the lateral penumbra of low-energy proton beams, brass collimators are often used in spot-scanning proton therapy (SSPT). This study investigates the increase in biological effectiveness through collimator scattered protons in SSPT. Methods: The SSPT system of the Hokkaido University Hospital Proton Beam Therapy Center, which consists of a scanning nozzle, a 2-cm thick brass collimator, and a 4-cm thick energy absorber, was simulated with our validated Geant4 Monte Carlo code (ver. 9.3). A water phantom was irradiated with proton pencil beams of 76, 110, and 143 MeV. The tested collimator opening areas (COA) were 5×5, 10×10, and 15×15 cm{sup 2}. Comparisons were made among the dose-averaged LET values of protons that hit the collimators (LETDColl), protons that did not hit the collimators (LETDNoColl), and all protons (LETDTotal). X-ray equivalent doses (Deq) were calculated using the linear-quadratic model with LETDNoColl and LETDTotal, and their maximum difference was determined over regions where the physical dose was greater than 10% of the peak dose of 2 Gy. Results: The ratio of the dose contribution of collimator scattered protons to that of all protons, defined as λ, was large at high proton energies and large COAs. The maximum λ value ranged from 3% (76 MeV, 5×5 cm{sup 2}) to 29% (143 MeV, 15×15 cm{sup 2}). Moreover, a large difference between LETDColl and LETDNoColl was only found in regions where λ was below 20% (ΔLETD > 2 keV/µm) and 8% (ΔLETD > 5 keV/µm). Consequently, the maximum difference between LETDNoColl and LETDTotal was as small as 0.8 keV/µm in all simulated voxels, and the difference of Deq reached a maximum of 1.5% that of the peak dose obtained at the water surface with a 76 MeV beam. Conclusion: Although collimator scattered protons have high LET, they only increase the physical dose, not the biological effectiveness.},
doi = {10.1118/1.4956267},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • High-energy protons traversing tissue produce local sources of high-linear-energy-transfer ions through nuclear fragmentation. The contribution of these target fragments to the biological effectiveness of high-energy protons using the cellular track model is examined. The effects of secondary ions are treated in terms of the production collision density using energy-dependent parameters from a high-energy fragmentation model. Calculations for mammalian cell cultures show that at high dose, at which intertrack effects become important, protons deliver damage similar to that produced by gamma rays, and with fragmentation the relative biological effectiveness (RBE) of protons increases moderately from unity. At low dose, where sublethalmore » damage is unimportant, the contribution from target fragments dominates, causing the proton effectiveness to be very different from that of gamma rays with a strongly fluence-dependent RBE. At high energies, the nuclear fragmentation cross sections become independent of energy. This leads to a plateau in the proton single-particle-action cross section, below 1 keV/micron, since the target fragments dominate. 29 refs.« less
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