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内容記述 |
FLASH radiotherapy has attracted attentions due to its advantages, sparing effects of healthy tissues for example. A lot of studies have been done in wide research fields, however, the underlying mechanisms of sparing effects by ultra-high dose rate irradiation have not been clarified. We have addressed to evaluate dose rate dependence of changes in yields of water radiolysis products. It is not feasible to experimentally obtain full datasets of water radiolysis species, so that in-silico studies are necessary to elucidate the mechanisms of dose rate effects from a radiation chemistry point of view.To do so, we have employed MPEXS-DNA based on Geant4 ver. 10.7. patch 4, which is a radiation simulator computing on NIVIDA GPU devices. MPEXS-DNA enables us to exactly reproduce an experimental condition, and to estimate yields of water radiolysis species using a step-by-step approach faster more than 1000 times relative to conventional Geant4-DNA simulations. Namely, MPEXS-DNA is one of the most suitable simulators for the simulation under ultra-high dose rates. Under protons, yields of OH radicals decrease with increasing dose rate (Figure 1). The trend is reasonable with experimental results. In hydrated electrons and hydrogen peroxide, significant changes in their yields are not observed with dose rate. With increasing dose rate, reactions of radiolysis species generated by a certain proton track with those by neighboring tracks occur significantly, leading to the reduction of reactive oxygen species (e.g., OH radicals). In addition, local and instantaneous reduction of oxygen concentration is seen. These findings suggest a decrease in indirect action and base damage. This would be one of the mechanisms of sparing effects under ultra-high dose rate irradiations. |
