Oxygen depletion hypothesis remains controversial: a mathematical model of oxygen depletion during FLASH radiation

TL;DR

This study presents a computational model analyzing oxygen depletion during FLASH radiation, finding that oxygen in normal tissues is unlikely to be depleted, thus challenging the oxygen depletion hypothesis as an explanation for the FLASH effect.

Contribution

The paper introduces a numerical model for oxygen dynamics during FLASH radiation, providing quantitative analysis that questions the validity of the oxygen depletion hypothesis.

Findings

Oxygen concentration change follows a negative exponential function.

High dose rates do not deplete oxygen more than lower dose rates for the same dose.

Model analysis does not support oxygen depletion as the mechanism behind FLASH effects.

Abstract

Background: Experiments have reported low normal tissue toxicities during FLASH radiation, but the mechanism has not been elaborated. Several hypotheses have been proposed to explain the mechanism. The oxygen depletion hypothesis has been introduced and mostly studied qualitatively. Methods: We present a computational model to describe the time-dependent change of oxygen concentration in the tissue. The kinetic equation of the model is solved numerically using the finite difference method. The model is used to analyze the FLASH effect with the oxygen depletion hypothesis, and the brain tissue is chosen as an example. Results: The oxygen distribution is determined by the oxygen consumption rate of the tissue and the distance between capillaries. The change of oxygen concentration with time after radiation has been found to follow a negative exponential function, and the time constant is determined by the distance between capillaries. When the dose rate is high enough, the same dose results in the same change of oxygen concentration regardless of dose rate. The analysis of FLASH effect in the brain tissue based on this model does not support the explanation of the oxygen depletion hypothesis. Conclusions: The oxygen depletion hypothesis remains controversial because oxygen in most normal tissues cannot be depleted by FLASH radiation according to the mathematical analysis with this model and experiments on the expression and distribution of the hypoxia-inducible factors.