A physicochemical model of reaction kinetics supports peroxyl radical recombination as the main determinant of the FLASH effect.

BACKGROUND AND PURPOSE

FLASH radiotherapy, a technique based on delivering large doses in a single fraction at the micro/millisecond timescale, spares normal tissues from late radiation-induced toxicity, in an oxygen-dependent process, whilst keeping full anti-tumor efficiency. We present a theoretical model taking into account the kinetics of formation and decay of reactive oxygen species, in particular of organic peroxyl radicals ROO formed by addition of O to primary carbon-centred radicals R and known to play a major role at the origin radio-induced complications.

MATERIALS AND METHODS

The model focuses on the time-dependent evolution of radiolytic products in living matter exposed to continuous irradiation at dose-rates in the range 10-10Gy·s. The 9 differential rate equations resulting from the radiolytic and enzymatic reactions network were solved using the published values of these reactions rate constants in a cellular environment.

RESULTS

The model suggests a correlation between the area-under-the-curve of time-evolving [ROO] and the probability of normal tissue complications. The model does not lend weight to the hypothesis of transient oxygen depletion as a main determinant of FLASH but rather suggests a major role of radical-radical recombination.

CONCLUSION

The model gives support to the reduction of ROO lifetime as the main root of FLASH and compares favorably with published experimental results. We conclude that any process - in this case radical recombination - that shortens the lifetime or limits the radiolytic yield of ROO is likely to protect normoxic tissues against the deleterious effects of radiation.

Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.