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Figure S1B- Higher magnification showing the skin of the mini-pig 15 and 32 weeks post radiotherapy (RT). At 15 weeks, skin area irradiated with 31Gy-FLASH-RT shows hair regrowth but not with 31Gy-Conv-RT. At 32 weeks, skin area irradiated with 34Gy-FLASH-RT is healthy whereas contractile skin fibrosis is observed with 34 Gy-Conv-RT.

Supplementary Data Table S1: WHO staging system (TNM Classification of Tumors in Domestic Animals. Geneva: World Health Organization; 1980) Table S2: Cat patient and treatment characteristics

Purpose: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.Experimental Design:Pig skin was used to investigate a potential difference in toxicity between irradiation delivered at an ultrahigh dose rate called “FLASH-RT” and irradiation delivered at a conventional dose rate called “Conv-RT.” A clinical, phase I, single-dose escalation trial (25–41 Gy) was performed in 6 cat patients with locally advanced T2/T3N0M0 squamous cell carcinoma of the nasal planum to determine the maximal tolerated dose and progression-free survival (PFS) 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 3 exhibited moderate/mild transient mucositis, and all cats had depilation. With a median follow-up of 13.5 months, the PFS at 16 months was 84%.Conclusions:Our results confirmed the potential advantage of FLASH-RT and provide a strong rationale for further evaluating FLASH-RT in human patients.See related commentary by Harrington, p. 3

Figure S1: A- Serial pictures showing evolution of the mini-pig's skin over time post FLASH and Conv-RT. Three doses are shown (28-34Gy) 24h to 32 weeks post-RT.