FLASH irradiation does not induce lipid peroxidation in lipids micelles and liposomes

Abstract

FLASH radiotherapy (FLASH-RT) enables the delivery of ultra-high dose rate (UHDR) radiation to a tumor, increasing the mean dose rate from 0.1 Gy· s−1of conventional radiotherapy (CONV-RT) to 100 Gy· s−1and above. Animal models have demonstrated that FLASH-RT preserves healthy tissues while yielding similar tumor growth delay as conventional irradiation. Despite the promise of FLASH-RT, the physico-chemical and biological mechanisms underlying the FLASH effect are still under investigation. Two mutually non-exclusive hypothesis have been proposed that could explain the FLASH effect: 1) a reduction in radical diffusion due to a higher recombination rate of primary radicals and, 2) a reduction in tissue oxygenation levels able to alter downstream responses to FLASH-RT. In this respect, lipid peroxidation is a chemical reaction consuming oxygen, which might be involved in the FLASH effect. Here we used linoleic acid micelles and phosphatidylcholine (PC) liposomes as a proxy for cell membrane to investigate the lipid peroxidation yield after irradiation with electrons in FLASH and CONV modalities. With this system, we measured significant differences in concentrations of lipid peroxidation endproducts between both modalities of irradiation. The lipid micelles and PC liposomes exhibited enhanced and linear dose-dependent levels of lipid peroxidation with CONV, while FLASH did not induce lipid peroxidation. Lowering the oxygen content from 21 to 4% resulted in a diminution of the lipid peroxidation yield in CONV, but not its complete suppression. The lipid peroxidation yield dropped rapidly when the dose per pulse was increased from 0.008 Gy·pulse−1to 10 Gy·pulse−1, with no lipid peroxidation occurring above 0.2 Gy·pulse−1. Our results are the first to identify the lack of lipid peroxidation after FLASH-RT, and point to lipids as potentially critical target in rationalizing possible mechanisms underlying the FLASH effect across multiple normal tissue sites.