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

• Published in: Medical physics (Lancaster) 2016-06, Vol.43 (6), p.3492-3492
• Main Authors: Matsuura, T, Takao, S, Matsuzaki, Y, Fujii, Y, Fujii, T, Maeda, K, Ueda, H, Koyano, H, Umegaki, K, Shirato, H
• Format: Article
• Language: English
• Subjects: 07 ISOTOPES AND RADIATION SOURCES; 60 APPLIED LIFE SCIENCES; Alloys; COLLIMATORS; DOSE EQUIVALENTS; EDUCATIONAL FACILITIES; KEV RANGE 01-10; MEV RANGE 10-100; MEV RANGE 100-1000; MONTE CARLO METHOD; Monte Carlo methods; PROTON BEAMS; Proton therapy; Protons; SIMULATION; Surface scattering; X RADIATION; X‐ray scattering
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.4956267

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 cm2. 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 cm2) to 29% (143 MeV, 15×15 cm2). 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.