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

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.