Assessment and profiling of FLASH-induced cellular DNA damage as mechanistic markers of the ‘FLASH effect’.

Several studies have reported the normal tissue-sparing effects of ultra-high dose rate ‘FLASH’ irradiation in vivo whilst maintaining tumour response, with an associated reduced damage burden also being proposed in vitro. FLASH exposures will generate higher initial concentrations of induced free radicals and this is proposed responsible for an early divergence of radiochemical events that could distinguish FLASH-RT from lower dose rate conventional-RT. Towards this, two key radiochemical mechanisms have been proposed to contribute to the sparing effects of FLASH: radical–radical recombination (RRR) and transient oxygen depletion (TOD), with both being proposed to lead to reduced levels of induced damage. In this study we demonstrate that FLASH irradiation at low oxygen tension induces lower levels of DNA damage in peripheral blood lymphocytes (PBLs) ex vivo, an effect modulated by oxygen tension, dose, and dose rate, supporting an oxygen-related mechanism contributing to the tissue-sparing effect of FLASH irradiation. Hypothesising that a likely outcome of RRR would be the formation of crosslink damage (via organic radical recombination), and that a likely outcome of TOD would be a more anoxic damage profile, we next examined the profile of FLASH-induced damage via Comet assay, assessing crosslink formation and/or anoxic DNA damage formation as putative markers of RRR and TOD respectively. Following FLASH irradiation, we see no evidence of any crosslink formation; however, FLASH irradiation induces a more anoxic profile of induced damage, supporting the TOD mechanism. Furthermore, treatment of PBLs pre-irradiation with BSO abrogates the reduced strand break damage burden mediated by FLASH exposures. In summary, we do not see any experimental evidence to support the RRR mechanism happening, but our data supports TOD as being a driver of the FLASH-mediated reduced damage burden. Finally, preliminary data is presented of tumour cell-specific damaging mechanisms that may account for FLASH’s effective tumour control whilst sparing normal tissue.