In vivo measurements of change in tissue oxygen level during irradiation reveal novel dose rate dependence

Background and purpose: This study aimed to investigate the radiochemical oxygen depletion (ROD) in vivo by directly measuring oxygen levels in various mouse tissues during ultra-high dose rate (UHDR) irradiation at clinically relevant doses and dose rates. Materials and methods: Mice bearing subcutaneous human glioblastoma (U-87 MG) tumors were used for tumor and normal tissue (skin, muscle, brain) measurements. An oxygen-sensitive phosphorescent probe (Oxyphor PtG4) was injected into the tissues, and oxygen levels were monitored using a fiberoptic phosphorometer during UHDR irradiation with a 6 MeV electron linear accelerator (LINAC). Dose escalation experiments (10-40 Gy) were performed at a dose rate of 1300 Gy/s, and dose rate escalation experiments were conducted at a fixed dose of 40 Gy with dose rates ranging from 2 to 101 Gy/s. Results: Radiation-induced change in tissue oxygenation (Delta pO(2)) increased linearly with dose and correlated with baseline tissue oxygenation levels in the range of 0 - 30 mmHg. At higher baseline tissue oxygenation levels, such as those observed in muscle and brain, there was no corresponding increase in Delta pO(2). When we modulated dose rate, Delta pO(2) increased steeply up to similar to 20 Gy/s and plateaued thereafter. The relationship between Delta pO(2) and dose rate showcases the interplay between ROD and reoxygenation. Conclusion: While UHDR irradiation induces measurable oxygen depletion in tissues, the observed changes in oxygenation levels do not support the hypothesis that ROD-induced radioresistance is responsible for the FLASH tissue-sparing effect at clinically relevant doses and dose rates. These findings highlight the need for further investigation into alternative mechanisms underlying the FLASH effect.