Oxygen Monitoring in Model Solutions and In Vivo in Mice During Proton Irradiation at Conventional and FLASH Dose Rates

FLASH is a high-dose-rate form of radiation therapy that has the reported ability, compared with conventional dose rates, to spare normal tissues while being equipotent in tumor control, thereby increasing the therapeutic ratio. The mechanism underlying this normal tissue sparing effect is currently unknown, however one possibility is radiochemical oxygen depletion (ROD) during dose delivery in tissue at FLASH dose rates. In order to investigate this possibility, we used the phosphorescence quenching method to measure oxygen partial pressure before, during and after proton radiation delivery in model solutions and in normal muscle and sarcoma tumors in mice, at both conventional (Conv) (similar to 0.5 Gy/s) and FLASH (similar to 100 Gy/s) dose rates. Radiation dosimetry was determined by Advanced Markus Chamber and EBT-XL film. For solutions contained in sealed glass vials, phosphorescent probe Oxyphor PtG4 (1 mu M) was dissolved in a buffer (10 mM HEPES) containing glycerol (1 M), glucose (5 mM) and glutathione (5 mM), designed to mimic the reducing and free radical-scavenging nature of the intracellular environment. In vivo oxygen measurements were performed 24 h after injection of PtG4 into the interstitial space of either normal thigh muscle or subcutaneous sarcoma tumors in mice. The g-value for ROD is reported in mmHg/Gy, which represents a slight modification of the more standard chemical definition (mu M/Gy). In solutions, proton irradiation at conventional dose rates resulted in a g-value for ROD of up to 0.55 mmHg/Gy, consistent with earlier studies using X or gamma rays. At FLASH dose rates, the g-value for ROD was similar to 25% lower, 0.37 mmHg/Gy. pO(2) levels were stable after each dose delivery. For normal muscle in vivo, oxygen depletion during irradiation was counterbalanced by resupply from the vasculature. This process was fast enough to maintain tissue pO(2) virtually unchanged at Conv dose rates. However, during FLASH irradiation there was a stepwise decrease in pO(2) (g-value similar to 0.28 mmHg/Gy), followed by a rebound to the initial level after similar to 8 s. The g-values were smaller and recovery times longer in tumor tissue when compared to muscle and may be related to the lower initial endogenous pO(2) levels in the former. Considering that the FLASH effect is seen in vivo even at doses as low as 10 Gy, it is difficult to reconcile the amount of protection seen by oxygen depletion alone. However, the phosphorescence probe in our experiments was confined to the extracellular space, and it remains possible that intracellular oxygen depletion was greater than observed herein. In cell-mimicking solutions the oxygen depletion g-vales were indeed significantly higher than observed in vivo. (C) 2022 by Radiation Research Society

Automated summary

FLASH is a form of high-dose-rate radiation therapy that can spare normal tissues while being effective in tumor control. The mechanism of normal tissue sparing effect in FLASH therapy may involve radiochemical oxygen depletion during dose delivery. This study used phosphorescence quenching method to measure oxygen partial pressure before, during, and after proton radiation delivery in solutions and in mice models. The results showed that the oxygen depletion was lower in FLASH dose rates compared to conventional dose rates. The recovery time was longer in tumor tissue, possibly due to the lower initial endogenous oxygen levels.