FLASH-RT does not affect chromosome translocations and junction structures beyond that of CONV-RT dose-rates

Background and purpose: The impact of radiotherapy (RT) at ultra high vs conventional dose rate (FLASH vs CONV) on the generation and repair of DNA double strand breaks (DSBs) is an important question that remains to be investigated. Here, we tested the hypothesis as to whether FLASH-RT generates decreased chromosomal translocations compared to CONV-RT.Materials and methods: We used two FLASH validated electron beams and high-throughput rejoin and genome-wide translocation sequencing (HTGTS-JoinT-seq), employing S. aureus and S. pyogenes Cas9 bait DNA double strand breaks (DSBs) in HEK239T cells, to measure differences in bait-proximal repair and their genome-wide translocations to prey DSBs generated after various irradiation doses, dose rates and oxygen tensions (normoxic, 21% O2; physiological, 4% O2; hypoxic, 2% and 0.5% O2). Electron irradiation was delivered using a FLASH capable Varian Trilogy and the eRT6/Oriatron at CONV (0.08-0.13 Gy/ s) and FLASH (1x102-5x106 Gy/s) dose rates. Related experiments using clonogenic survival and cH2AX foci in the 293T and the U87 glioblastoma lines were also performed to discern FLASH-RT vs CONV-RT DSB effects. Results: Normoxic and physioxic irradiation of HEK293T cells increased translocations at the cost of decreasing bait-proximal repair but were indistinguishable between CONV-RT and FLASH-RT. Although no apparent increase in chromosome translocations was observed with hypoxia-induced apoptosis, the combined decrease in oxygen tension with IR dose-rate modulation did not reveal significant differences in the level of translocations nor in their junction structures. Furthermore, RT dose rate modality on U87 cells did not change cH2AX foci numbers at 1-and 24-hours post-irradiation nor did this affect 293T clonogenic survival.Conclusion: Irrespective of oxygen tension, FLASH-RT produces translocations and junction structures at levels and proportions that are indistinguishable from CONV-RT.(c) 2023 Elsevier B.V. All rights reserved. Radiotherapy and Oncology 188 (2023) 109906

Automated summary

The study aimed to investigate the effects of radiotherapy at different dose rates on DNA double strand breaks. The results showed that both ultra high dose rate and conventional dose rate radiotherapy produced chromosomal translocations, and the levels and structures were not significantly different from those produced by conventional dose rate radiotherapy.