Comparative Evaluation of the Volumetric Effects on Chemical Yield in Monte Carlo Computational Approaches

Purpose: This study aimed to investigate how chemical yields depend on simulating volume size for three chemistry-stage simulation models available in Geant4-DNA, i.e., the step-by-step (SBS), independent reaction time (IRT), and SBS-reaction-diffusion master equation (RDME) models, using the "UHDR" example.Methods: Simulations were conducted using the Geant4-DNA "UHDR" example, which were modified to enable calculations using all three models.A planar 1 MeV electron beam was incident on cubic water phantoms with side lengths ranging from 3.2 to 102.4 μm for the SBS and IRT methods and from 2.4 to 4.0 μm for the SBS-RDME method.The chemical yields of H 3 O + , OH, H 2 , e - aq , H, and H 2 O 2 were evaluated up to 1 μs.The SBS-RDME approach accounts for intertrack effects by accumulating the dose to predefined thresholds before initiating the chemical stage.Results: For both the IRT and SBS models, increasing the simulation volume resulted in decreased yields of reactants (H 3 O + , OH, and e - aq ) and increased yields of products (H 2 , H, and H 2 O 2 ) primarily due to the enhanced contribution of low-energy secondary electrons.In the SBS model, small volumes exhibited a distinct drop in yields between 10 -8 and 10 -6 seconds due to species escaping through boundaries.Compared with the IRT model, the SBS model yielded higher G values at 1 μs for most species except H 2 .For the SBS-RDME simulations with intertrack effects, the chemical yields were more sensitive to dose thresholds than to volume.Species reflection at boundaries prevented sharp yield decreases, while higher dose threshold values enhanced recombination.Conclusions: We simulated chemical yield variations using three calculation models while varying the volume of interest.This study can serve as a reference for researchers using Geant4-DNA and Monte Carlo track-structure simulations to design appropriate geometric configurations.