FLASH irradiation induces lower levels of DNA damage ex vivo, an effect modulated by oxygen tension, dose, and dose rate

Abstract

Objective: FLASH irradiation reportedly produces less normal tissue toxicity, while maintaining tumour response. To investigate oxygen’s role in the ‘FLASH effect’, we assessed DNA damage levels following irradiation at different oxygen tensions, doses and dose rates. Methods: Samples of whole blood were irradiated (20 Gy) at various oxygen tensions (0.25–21%) with 6 MeV electrons at dose rates of either 2 kGy/s (FLASH) or 0.1 Gy/s (CONV), and subsequently with various doses (0–40 Gy) and intermediate dose rates (0.3–1000 Gy/s). DNA damage of peripheral blood lymphocytes (PBL) were assessed by the alkaline comet assay. Results: Following 20 Gy irradiation, lower levels of DNA damage were induced for FLASH, the difference being significant at 0.25% (p < 0.05) and 0.5% O2 (p < 0.01). The differential in DNA damage at 0.5% O2 was found to increase with total dose and dose rate, becoming significant for doses ≥20 Gy and dose rates ≥30 Gy/s. Conclusion: This study shows, using the alkaline comet assay, that lower levels of DNA damage are induced following FLASH irradiation, an effect that is modulated by the oxygen tension, and increases with the total dose and dose rate of irradiation, indicating that an oxygen related mechanism, e.g. transient radiation-induced oxygen depletion, may contribute to the tissue sparing effect of FLASH irradiation. Advances in knowledge: This paper is first to directly show that FLASH-induced DNA damage is modulated by oxygen tension, total dose and dose rate, with FLASH inducing significantly lower levels of DNA damage for doses ≥20 Gy and dose rates ≥30 Gy/s, at 0.5% O2.

Treatment planning considerations for the development of FLASH proton therapy

“…This 'breaking' behaviour, in which cellular sensitivity starts to follow an anoxic-like response after a certain dose is reached, provides evidence for radiolytic oxygen depletion being responsible for the protective effects of ultra-high-dose-rate radiation. More recent in vitro studies have also demonstrated a dose-dependent sparing effect which, for low oxygen conditions (<2% O 2 ), becomes significant at 18-20 Gy, thus corroborating this theory [30,31,32]. Following the radiolytic oxygen depletion hypothesis, a dose threshold governed by initial levels of oxygen to deplete for an observable shift in radiosensitivity, becomes an integral requirement for FLASH.…”

“…Evidence for a dose threshold has so far generally been limited to in vitro studies [30,31,32,34]. In vivo, the requirement for a sufficiently high dose to generate normal tissue toxicity limits the range of doses that can be used in investigations.…”

Practice-oriented solutions integrating intraoperative electron irradiation and personalized proton therapy for recurrent or unresectable cancers: Proof of concept and potential for dual FLASH effect

“…The irradiation times in FLASH-RT is 400-fold shorter than that in conventional RT ( 21 ). The FLASH effect (i.e., sparing of normal tissue) has been observed with both electron ( 19 , 20 , 22 – 26 ) and proton beams ( 27 – 31 ) in animal experiments. Tumor control has been observed to be equivalent to conventional RT, although the number of studies is limited ( 19 , 29 , 31 , 32 ).…”

Ultra-high dose rate FLASH irradiator at the radiological research accelerator facility

“…In lymphocytes, Cooper et al 39 saw no difference in comet formation between FLASH and conventional dose rate, however, in their experiments, lymphocytes were analyzed immediately after irradiation. Their model may therefore lack some live cell features that may lead to a "FLASH effect" (e.g., oxygen metabolism and DNA damage processing), compared to our experimental setup, where cells were allowed to perform DNA repair for 48 h in culture.…”