Author: Yujie Chi, Yao Hao, Xun Jia, Youfang Lai, Yuting Peng, Francisco Javier Reynoso, Lingshu Yin, Tianyu Zhao, Xiandong Zhao 👨🔬
Affiliation: Johns Hopkins University, Washington University School of Medicine, Department of Radiation Oncology, Washington University School of Medicine, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University, Department of Physics, University of Texas at Arlington, Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, University of South Florida 🌍
Purpose: FLASH radiation therapy has shown remarkable tissue sparing effects compared to radiation therapy at conventional dose rates (CDR). Yet, its mechanism remains unclear. Radical production modulated by dose rate and oxygen level is expected to be factors triggering different radiobiological responses. This study investigates the impacts of dose rate and oxygen level on radical yields under proton FLASH irradiation via two radiochemical Monte Carlo (MC) models.
Methods: An in-house developed GPU-based microscopic MC simulation package (gMicroMC) was used to model radiation physics and chemical processes with a periodic boundary condition. Temporal evolution of the radical yields was computed, and radical yields with different dose rates (CDR and UHDR) were examined as well. Another Geant4-DNA based model was performed to investigate the radical yields under real proton FLASH irradiation by simulating a real single pulse in a pencil beam scanning system. Both models considered the inter-track interactions of radicals generated from different proton tracks.
Results: Under the proton FLASH conditions, radical production was altered as a result of the spatial and temporal overlap of radicals produced by different protons and the mutual reactions among them. For the gMicroMC simulation of 142.4 MeV protons, at the end of 50 micropulses, the chemical yield of OH· under FLASH scheme was decreased by ∼14% compared with that under the CDR condition, while Geant4 simulation showed a significant ~76% reduction in hydroxyl after 2.5 mSec for the FLASH proton irradiation of ~30MeV.
Conclusion: Both MC models demonstrated the capability of simulating the realistic scenario under proton FLASH irradiation, and the UHDR proton beams result in a significant reduction in radical yields.