Investigating radical yield variations in FLASH and conventional proton irradiation via microscopic Monte Carlo simulations

Abstract Objective: Ultra-high-dose rate (UHDR) FLASH radiation therapy has shown remarkable tissue sparing effects compared to that at conventional dose rates (CDR). Radical production modulated by dose rate is expected to be one of the factors triggering different radiobiological responses. This study investigates the impacts of dose rate on radical yields in UHDR FLASH and CDR proton irradiation via GPU-based microscopic Monte Carlo (MC) simulations. 
Approach: We considered a region of interest (ROI) irradiated by a proton beam produced with a synchrotron pulse structure. The number of protons entering into the ROI was estimated in UHDR and CDR conditions. We sampled protons entering the ROI with randomly distributed spatial and temporal positions. An in-house developed GPU-based microscopic MC simulation package was used to model radiation physics and chemical processes with a periodic boundary condition. The temporal evolution of the radical yields was computed for different radical types, which in this work are hydrated electron eh, hydroxyl ·OH, hydrogen radical H· and hydrogen peroxide H2O2. We also examined radical yields with different proton energies from 1 to 142.4 MeV. 
Main results: Under the UHDR FLASH conditions, radical production was altered as a result of the spatial and temporal overlap of radicals produced by different protons, causing a change in their interactions. For the case with 142.4 MeV protons after 50 micropulses, the chemical yield of ·OH under the FLASH scheme was decreased by ∼14% compared with that under the CDR condition. The percentage of reduction increased with the number of micropulses and decreased with proton energy.
Significance: We modeled microscopic phenomena of radiation physics and chemistry triggered by synchrotron proton irradiation under UHDR FLASH and CDR conditions. Our results provided insights into the underlying mechanisms responsible for the FLASH effect.