Impact of increased oxygen concentration on the FLASH sparing effect in mice is tissue dependent

OBJECTIVES: Previous findings reported increased toxicity of FLASH radiotherapy (FLASH) delivered under supplemental oxygen but failed to quantify the change in magnitude of the FLASH sparing effect. This study investigated the impact of oxygen breathing on normal tissue complication probability (NTCP) following FLASH and conventional radiotherapy (CONV) using murine models of acute gastrointestinal and skin toxicity. METHODS: Tumour-free C57BL/6 and BALB/c mice received whole-abdominal or skin irradiation, respectively, using either CONV or FLASH. Dose escalation was performed under air- and oxygen-breathing in both models to determine NTCP curves. The FLASH dose modifying factor (DMF) was derived as the dose ratio at 50% NTCP. Changes in tissue oxygenation, going from air- to oxygen-breathing, were measured in vivo using oxygen sensing by phosphorescence quenching. RESULTS: Abdominal irradiation under air breathing showed a significant FLASH sparing effect, with a mean DMF of 1.14. This effect was abolished under oxygen breathing, with a mean DMF reduced to 1.00. In contrast, skin irradiation under air breathing demonstrated a strong FLASH sparing effect, with a mean DMF of 1.42, which was largely preserved under oxygen breathing (mean DMF = 1.40). Oxygen breathing markedly increased tissue oxygenation in both the intestine and skin. CONCLUSIONS: Maintaining of the FLASH sparing effect under increased oxygenation is tissue dependent. Our findings indicate potential difference in CONV and FLASH oxygen enhancement ratio (OER) curves. ADVANCES IN KNOWLEDGE: By conducting dose escalation experiments in animal models we provided first quantification of change in magnitude of FLASH sparing effect under increased tissue oxygenation.