Ultra-High Dose-Rate Oxygen Depletion and Skin Response to Irradiation

Background/Objectives: This study investigates the hypothesis that transient oxygen depletion is the mechanism of the skin sparing effect of ultra-high dose-rate irradiation, commonly referred to as FLASH irradiation. Methods: Two skin tattoo dots were placed approximately 1.0 cm apart on the thigh of FVB/N mice. The area overlapping the dots was irradiated with a single dose of 27 Gy protons delivered with either FLASH (~120 Gy/s) or 0.5 Gy/s conventional dose-rate (CDR) irradiation. Skin contraction was assessed by measuring the distance between the tattoo dots and complemented by histopathological skin analyses. Mice were placed in a 1.4 L chamber flushed with 5%, 7%, 20.9% or 100% oxygen (balance nitrogen, where applicable) prior to and during irradiation. Skin oxygenation was measured non-invasively using the phosphorescence quenching method. Results: Compared to air-breathing mice, skin contraction increases in mice breathing 100% oxygen and decreases when breathing 7% and 5% oxygen following CDR irradiation, showing that skin is neither fully oxygenated nor hypoxic. FLASH irradiation reduced skin contraction, epidermal thickening, and fibrosis in air-breathing mice compared to CDR irradiation. The difference between FLASH and CDR skin contraction decreases as the inspired gas oxygen content is reduced from 20.9% to 7%. Under 5% oxygen breathing conditions, the FLASH sparing effect is eliminated. Conclusions: Mean normal tissue pO₂ does not reveal the presence of cells at low pO₂ that could become susceptible to FLASH-induced radiobiological hypoxia at doses lower than would be predicted from the mean tissue pO₂ value. In the absence of oxygen, FLASH skin sparing for the late normal tissue effect, skin contraction, is eliminated.