Breathing new life into old theory.
Though theoretical models have upheld oxygen depletion as a viable explanation for the unprecedented therapeutic index achieved by FLASH-RT, as with many things rad bio, no one really knows. Recent reporting of in vivo experiments show the total decrease in oxygen after 20 Gy x 1 delivered conventionally (at a rate of 0.1 Gy/sec) to both normal and tumor tissue resulted in no measurable decrease in tissue oxygenation. On the other hand, 20 Gy x 1 delivered at FLASH (a rate approaching 300 Gy/sec) resulted in measurable decreases in oxygenation of 2.3±0.3 mmHg in normal tissue and 1.0±0.2 mmHg in tumor tissue. Great, so we’ve finally figured it out! Well, again not really, because the authors conclude such levels of oxygen depletion are unlikely to be clinically relevant. | Cao, Int J Radiat Oncol Biol Phys 2021