Fixation method influences FLASH skin sparing in an in vivo leg model.

Background and purpose: The FLASH effect, where ultra-high dose rate elicits a favourable normal tissue-sparing, has been shown in several preclinical studies. Study setup differences, for example fixation methods that affect blood flow, can influence radiation response but are unexplored for FLASH. This study compared FLASH's acute skin-sparing effect with two fixation methods: a glued fixation (no blood flow restriction) and taped fixation (slight blood flow restriction). Patient/material and methods: Female CDF1 mice were irradiated on their hind foot using a glue-fixation or tape-fixation method. Glue-fixated mice were only taped during the glueing procedure and had a 10-min unrestricted period afterwards before irradiation, while tape-fixated mice were taped shortly before and throughout irradiation. Mice received single-dose irradiation (19-58 Gy) with either conventional dose rate (CONV, protons 0.06 Gy/s, electrons 0.16 Gy/s) or FLASH (electrons, 223-233 Gy/s). Differences in skin toxicity were analysed.

Results: CONV-treated tape-fixated mice required a 16-17% higher dose to induce skin toxicity relative to glued mice for both protons and electrons. Meanwhile, the fixation method did not affect FLASH-treated mice. The resulting electron FLASH-sparing effect was reduced by 18% due to the shift in radiosensitivity for CONV-treated mice.

Interpretation: CONV-treated tape-fixated mice were more radioresistant than the glue-fixated mice, consistent with the expected response to mild hypoxia. FLASH-treated mice were unaffected. These findings demonstrate the impact of fixation and, in turn, oxygen level on the differential CONV versus FLASH skin response. The results highlight the importance of minimal systemic influence on animals during FLASH studies.