FLASH effect tissue sparing comparison for X-ray, hadron, and VHEE irradiation modalities

Abstract

Abstract FLASH-RT is an irradiation modality using Ultra-High-Dose-Rates, where a healthy tissue sparing effect is observed. Different particle types have distinct characteristics in lateral and distal dose deposition, which can affect the amount of healthy tissue sparing by the FLASH effect. However, few comparative studies investigate this impact. In this Monte Carlo-based study, we compare different irradiation modalities in a simple geometrical model and demonstrate how dose, dose-rate, and particle type influence the volume of healthy tissue benefiting from FLASH. The results indicate that less conformal modalities, such as X-rays and VHEE, benefit more from the effect than hadrons. Despite this, FLASH X-rays and FLASH VHEE still deposit higher dose to healthy tissue than conventional hadrons. Assuming a FLASH-triggering dose threshold and dose-rate threshold from literature, we found that hadrons require a target dose of 20 Gy and target dose-rate of 75 Gy/s to achieve healthy tissue sparing, about twice as high as for VHEE and X-rays. The results imply that highly fractionated treatments cannot be applied under current assumptions of FLASH-triggering dose and dose-rate thresholds. Future studies using clinical dose distributions could explore the use of multiple angles, fractionation possibilities, and relative biological effectiveness (RBE) for more specific clinical cases.