Ultra-Fast Oxygen Monitoring During Proton FLASH Delivery Using the Phosphorescence Quenching Method and Soluble Oxyphor Probes.

Abstract

PURPOSE/OBJECTIVE(S)\nFLASH is a promising form of radiation therapy due to its reported ability to spare normal tissues while being equipotent in tumor control. The mechanism underlying this tissue sparing effect is currently unknown, however the depletion of tissue oxygen at FLASH dose rates has been proposed as a potential source. To date, the oxygen depletion hypothesis has only been supported by analytical or numerical modeling studies, primarily due to the difficulty of measuring oxygen concentration on millisecond time scales during irradiation. In this project, we use the phosphorescence quenching method to probe oxygen depletion during proton delivery at both conventional (∼1Gy/sec) and FLASH (∼100 Gy/sec) dose rates.\n\n\nMATERIALS/METHODS\nOxygen concentration was measured at rates of up to 3kHz using a phosphorescent probe, Oxyphor PtG4, and a time-domain OxyLED phosphorometer modified for ultra-fast operation. Sealed glass vials, containing the experimental solution and probe, were placed such that they were exposed to a uniform dose rate where the proton beam entered the vial. Radiation was delivered at conventional (conv) (∼1 Gy/s) and FLASH (∼100 Gy/s) dose rates. The g-value of oxygen depletion was measured as a function of dose rate, total dose, oxygen concentration, biomolecule concentration, and in the presence or absence of oxygen scavengers. Biomolecule relevant experiments were carried out using either 20mM phosphate buffer containing 1, 2, and 5% bovine serum albumin (BSA) or a solution designed to emulate the intracellular environment, containing 5mM glucose, 1M glycerol, 5mM glutathione, and 10 mM HEPES (Buffer 3G). Oxygen concentration was continuously monitored before, during, and after irradiation for all experiments.\n\n\nRESULTS\nWe found that FLASH dose rates resulted in lower g-values for oxygen depletion when compared to conventional dose rates by ∼15% (range: 7.2 to 22.5%) for each of the solutions tested. For example, in Buffer 3G, for a 30Gy dose, the g-value was 16% lower at FLASH dose rates (gconv\u202f=\u202f0.72 ± 0.02 μM/Gy, gFLASH\u202f=\u202f0.63 ± 0.02 μM/Gy) (all values listed are mean ± S.E.M.). The g-value increased with increasing BSA concentration (values for 1% and 5% BSA listed) at both conventional and FLASH dose rates (gconv\u202f=\u202f0.45 ± 0.05 and 0.58 ± 0.03 μM/Gy respectively, gFLASH\u202f=\u202f0.37 ± 0.01 and 0.62 ± 0.01 μM/Gy respectively). The g-value was higher for Buffer 3G than for the BSA solutions (gconv\u202f=\u202f0.72 ± 0.02 and 0.58 ± 0.03 μM/Gy respectively, gFLASH\u202f=\u202f0.63 ± 0.02 and 0.54 ± 0.01 μM/Gy respectively). An after FLASH period of gradual oxygen depletion which persisted for ∼0.5s and depleted an additional 32.1% oxygen was observed for Buffer 3G.\n\n\nCONCLUSION\nWe have demonstrated a method for ultra-fast measurements of oxygen concentration during the delivery of proton radiation. Furthermore, we found that the g-value of oxygen depletion at FLASH dose rates is lower than that of conventional dose rates by approximately 15%.