Hypoxic tumors are sensitive to FLASH radiotherapy

Tumor hypoxia is a major cause of resistance to cancer treatments and especially to radiotherapy (RT) and we wanted to assess whether ultra-high dose rate FLASH RT could overcome this resistance. We engrafted tumor cells of various origins subcutaneously in mice to provide a reliable and rigorous way to modulate oxygen supply via vascular clamping or carbogen breathing. We irradiated tumors using a single 20 Gy fraction at either conventional (CONV) or FLASH dose-rate. Using multiple different subcutaneous tumor models, and in contrast CONV-RT, FLASH-RT retained anti-tumor efficacy under extreme hypoxia. These findings demonstrate that in addition to normal tissue sparing, FLASH-RT overcomes hypoxia-mediated tumor resistance. Follow-up molecular analysis using RNAseq profiling uncovered FLASH-specific inhibition of cell proliferation and translation as well as metabolic shifts that discriminated FLASH-RT from CONV-RT. These data provide new and specific insights into the mechanism of action and identify novel targets for intervention. ### Competing Interest Statement The authors have declared no competing interest.