Impact of Microdosimetric Energy Deposition on Radiolytic Chemistry
Abstract
Purpose
Proton therapy treatments deliver a prescribed absorbed dose to the patient. This work investigates both energy deposition and the temporal evolution of water radiolysis products at micrometric scales, combining a multi-scale approach to assess how identical absorbed doses can correspond to different energy deposition patterns and chemical environments.
Methods
We used a detailed model of the Hitachi proton therapy nozzle in Open TOPAS to simulate a 10×10 cm² homogeneous field. The integrated depth–dose (IDD) in water was calculated twice for each energy, modifying the number of incident protons to ensure the same absorbed dose in two scoring volumes: in the plateau and in the Bragg peak. Dose was scored in a macroscopic cube (0.6mm lateral size), and lineal energy in a subvolume (50µm lateral size) centered within it using TOPAS-nBio. Water radiation-induced chemistry was tracked in a cubic subvolume (10µm lateral size) within the microdosimetric region, scoring the temporal evolution of radiolysis products such as hydroxyl radicals and hydrogen peroxide.
Results
Microscopic energy depositions in the Bragg peak are denser but of lower maximum energy than in the plateau. This is reflected in the larger area under the y·d(y) weighted lineal energy distribution curve, indicating higher density of low-energy depositions. These differences influence chemical kinetics: highly reactive species such as hydroxyl radicals decay faster at the Bragg peak due to enhanced recombination, whereas more stable molecules such as hydrogen peroxide accumulate faster.
Conclusion
Monte Carlo simulations are a valuable tool to show how the same dose can correspond to different distributions of lineal energy and radiolytic chemical dynamics. An accurate quantification of these processes could set the basis for an understanding of the biological effectiveness of proton therapy.