The mechanism(s) driving the enhanced therapeutic ratio of ultra-high dose-rate radiotherapy (FLASH-RT) over slower conventional (CONV) ionizing radiation (IR) dose-rate are not known. However, attenuated DNA damage and transient oxygen depletion are among a number of proposed models. In this study, we employ high-throughput genome-wide translocation sequencing (LAM-HTGTS), a highly-sensitive assay used to identify “prey” DNA double strand breaks (DSBs) via their translocation to a single CRISPR-Cas9 “bait” DSB break, to investigate the potential differences in DSBs and repair generated by 10 Gy delivered at CONV (0.13 Gy/s) or FLASH (5.106 Gy/s) dose rates with the eRT6 (PMB-Alcen, CHUV). Investigations were conducted under different oxygen tensions. IR exposure in HEK293T cells at normoxic (21% O2) conditions increased both the proportion and total number of translocations from the junctions recovered, as expected, but were indistinguishable between CONV-RT and FLASH-RT dose-rates. Although, increased proportions of translocations were observed as cells were acutely transitioned to either physioxic (4%) or hypoxic (<2%) conditions alone, the combined decrease in oxygen tension with IR dose-rate modulation did not reveal overt differences in the level of increased translocations nor in junction structures, which were predominantly increased in microhomology utilization. We discuss these findings with other related LAM-HTGTS studies.

Citation Format: Paul Barghouth, Jonathan Ollivier, Pierre Montay-Gruel, Billy W. Loo Jr., Marie-Catherine Vozenin, Charles Limoli, Richard Frock. Ultra-high dose rate (FLASH) irradiation does not alter microhomology mediated recombination under varying oxygen tension when compared to standard clinical dose rates [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-012.