Background: Previous studies using FLASH radiotherapy (RT) in mice showed a marked increase of the differential effect between normal tissue and tumors. To stimulate clinical transfer, we evaluated whether this effect could also occur in higher mammals. Methods: Pig skin was used to investigate a potential difference in toxicity between irradiation delivered at ultra-high dose rate called FLASH-RT and irradiation delivered at conventional dose rate called Conv-RT. A clinical-phase-I single-dose escalation trial (25-41 Gy) was performed in six cat-patients with locally-advanced T2/T3N0M0 squamous-cell-carcinoma of the nasal planum, to determine the maximal tolerated dose and progression-free survival of single-dose FLASH-RT. Results: Using respectively depilation and fibronecrosis as acute and late endpoints, a protective effect of FLASH-RT was observed (≥ 20% dose-equivalent difference vs Conv-RT). Three cats experienced no acute toxicity, whereas three exhibited moderate/mild transient mucositis and all cats had depilation. With a median follow-up of 13.5 months, the progression free survival (PFS) at 16 months was 84%. Conclusion: Our results confirmed the potential advantage of FLASH-RT and provide a strong rational for further evaluating FLASH-RT in human patients.
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