Impact of Nonlinear Absorption on Energy Distribution During FLASH-Like Deposition

FLASH radiotherapy for tumors has proven highly effective in preserving normal tissue; however, the physical origin of its biological selectivity has yet to be elucidated. This study developed a minimal diffusion-absorption model to examine the effects of nonlinear energy absorption on the spatial distribution of energy during FLASH-like deposition. Simulations revealed that nonlinear absorption alters the transfer of energy through the system while reducing the amount of energy absorbed in surrounding, unexposed regions. This behavior qualitatively resembles the tissue-sparing effect observed in FLASH radiotherapy. Parameter-space analysis revealed that the effect of incident pulse intensity depends on the absorption properties of the material, including its saturation behavior and energy threshold. These results suggest that energy-deposition conditions should be matched to the absorption characteristics of the target material.