Intermediate tissue oxygen level is required to observe murine FLASH skin sparing

This study evaluated the hypothesis that baseline tissue oxygen (pO₂) would modulate FLASH damage sparing in murine skin, comparing MeV ultra-high dose rate (UHDR) versus conventional dose rate (CDR).

Murine leg skin pO₂ was systematically varied and measured during irradiation from a Mobetron 9 MeV linac at 25 Gy, comparing UHDR (≈240 Gy/s) to CDR (≈0.16 Gy/s), for skin damage outcomes. Radiolytic oxygen consumption, g_O2 (mmHg/Gy), was also quantified in vivo. Baseline tissue pO₂ was systematically modulated in 5 different treatment cohorts, using known methods of the inhaled gas (room air, 100% oxygen, or carbogen) and through applied limb vascular compression (partial or full). Induced skin damage was scored daily per mouse, and averaged in each group.

FLASH skin sparing was observed in groups with partial leg clamping (pO₂≈7±4mmHg), inhaled air (pO₂≈12±6mmHg) and oxygen (tissue pO₂≈16±4mmHg), while reduction in time-to-damage was significant just in the air inhalation group. No FLASH effect was observed at zero oxygen, via complete blood flow occlusion (pO₂≈0±1mmHg), or when modulated by inhaled carbogen (pO₂≈21±7mmHg). In vivo measure of radiolytic consumption, g_O2, correlated to initial pO2 when FLASH was present (pO₂≈4-16mmHg) and saturated above pO₂>16mmHg. Inspired carbogen induced highest the pO₂ and maximum damage at 25 Gy. Reductions in dose to 20 and 15 Gy reduced skin damage, but did not result in FLASH sparing.

These findings indicate that tissue pO₂ directly modulates in vivo FLASH skin tissue sparing, requiring roughly room air or 100% oxygen anesthetic gas levels for this effect to be present. The FLASH effect is diminished by the absence of oxygen or from abnormally high oxygen values. Variations in dose does not appear to alter this observation at carbogen induced pO₂ levels, and the consumption rate of oxygen is directly correlated to the baseline value when the FLASH effect is present.