Author: Yao Hao, Yuting Peng, Francisco Javier Reynoso, Tianyu Zhao, Xiandong Zhao 👨🔬
Affiliation: Varian, University of South Florida, Washington University School of Medicine, Washington University School of Medicine, Department of Radiation Oncology 🌍
Purpose: Ultra-high dose-rate (FLASH) dose rate (> 40 Gy/s) may result in a radio-protective effect in healthy tissue compared with conventional dose rate (CDR). This effect has been observed in vivo and while several hypotheses have been proposed, the mechanism is not well-understood. This study investigates the impacts of oxygen level on radical yields under proton FLASH irradiation via a Geant4-based radiochemical Monte Carlo (MC) model.
Methods: Geant4-DNA has the capability to simulate the physicochemical and chemical stages by generating the free radicals and the reactions between them. A water phantom with an initial O2 density was irradiated by a proton beam with different dose rates by varying the number of protons simulated in the chemical stage. The inter-track interactions of radicals generated from different proton tracks were considered in our model, which is not an option in Geant4-DNA and added explicitly. The effects of different O2 level concentrations were studied by scoring the different radical yields at physoxia conditions.
Results: A significant reduction of the different radical yields under proton FLASH irradiation was found in our simulation. For the case with 200 Gy/s dose rate, at the end of two pulses (2.5 ms), the yield of OH· has 74% reduction compare with CDR mode of the same proton machine (8 Gy/s). The presence of oxygen impacts all the radicals investigated significantly, even in the physoxia 1% condition (8mmHg, 9.6uM): The yields of OH· and H show 17% and 20% reduction under FLASH irradiation, respectively. On the contrary, the yield of H2O2 increases ~6%.
Conclusion: The simulation of our G4-DNA based MC model indicates that the oxygen concentration impacts the radical yields significantly. The implementation of this model can be used to investigate the FLASH effect.