Speakers
Descrizione
In phenomenological stochastic models of the biological cellular effect of ionizing radiation, e.g. the multiple target multiple hit model (MTMH), one uses the concept of threshold - the effect happens if at least M critical cellular targets are hit by at least N ionizing particles each, the effect doesn’t happen otherwise. In these models, a notable sigmoidal shape of the biological effect versus absorbed dose curve originates from the spatial inhomogeneity of the cellular regions hit by the radiation - some regions are hit many times and contribute to the possible cellular effect, the others do not. The active radicals and ions created by the ionizing particles and eventually (directly or indirectly) causing the biological damage also have spatially inhomogeneous concentration or, in other words, mesoscopic spatial fluctuations around the average value. The fluctuations of the radial concentration dissipate due to the diffusion and the recombination processes in competition with creation of new radicals due to dose deposition at a given dose rate, and this kinetics affects the eventual biological effect.
We present a simple theory of the diffusive relaxation of mesoscopic, non-thermodynamic fluctuations of radical concentration and its application to the dose rate (FLASH) effect in radiotherapy. The conclusions of the model are in qualitative agreement with the recent review [1] of the available experimental results of the FLASH sparring effect for normal tissue complications probability. In particular the model predicts the increase of the FLASH effect at higher doses and also the threshold (minimal dose) below which the FLASH sparring is not observed.
[1] Böhlen TT, Germond JF, Bourhis J, Vozenin MC, Ozsahin EM, Bochud F, Bailat C, Moeckli R. Normal Tissue Sparing by FLASH as a Function of Single-Fraction Dose: A Quantitative Analysis. Int J Radiat Oncol Biol Phys. 2022 Dec 1;114(5):1032-1044. doi: 10.1016/j.ijrobp.2022.05.038. Epub 2022 Jul 8. PMID: 35810988.
