Impact of oxygen deprivation on the FLASH effect for skin toxicity in a murine model.

Abstract

FLASH radiotherapy has emerged as a promising modality capable of reducing normal tissue toxicity while maintaining tumor control comparable to conventional dose rate (CONV) radiotherapy. Despite growing interest, the underlying biological mechanisms remain unclear. This study investigates the dependency of FLASH irradiation on the tissue oxygen level by evaluating the skin-sparing effect of FLASH irradiation under normoxic and experimentally induced hypoxic conditions in a validated murine model. The right hindleg of unanesthetized female C3H/HeNRj mice was irradiated using either CONV (0.16 Gy/s) or FLASH (231.4 Gy/s) under normoxic or hypoxic conditions. Hypoxia was induced by clamping to hinder blood flow to the irradiated leg at least ten minutes before- and during irradiation. Irradiation was delivered using a FLASH-enabled linear accelerator with a 16 MeV electron beam. Acute radiation response was assessed daily via a graded skin toxicity scoring system to generate dose-response curves. Under induced hypoxia, oxygen enhancement ratios were 1.38 for FLASH and 2.05 for CONV, consistent with hypoxia-induced radioresistance, demonstrating a normal tissue-sparing effect. The FLASH dose-modifying factor (DMF) was 1.43 in normoxia and 0.96 in hypoxia, indicating a reversed FLASH effect under hypoxic conditions. Our findings suggest that oxygen availability is required for the normal tissue sparing associated with FLASH irradiation, and during hypoxia the FLASH DMF was reversed.