Geant4-DNA Modeling of Water Radiolysis beyond the Microsecond: An On-Lattice Stochastic Approach.

Hoang Ngoc Tran,Flore Chappuis,Laurent Desorgher,Sébastien Incerti,Francois Bochud

doi:10.3390/ijms22116023

Abstract

In this work, we use the next sub-volume method (NSM) to investigate the possibility of using the compartment-based (“on-lattice”) model to simulate water radiolysis. We, first, start with a brief description of the reaction-diffusion master equation (RDME) in a spatially discretized simulation volume (“mesh”), which is divided into sub-volumes (or “voxels”). We then discuss the choice of voxel size and merging technique of a given mesh, along with the evolution of the system using the hierarchical algorithm for the RDME (“hRDME”). Since the compartment-based model cannot describe high concentration species of early radiation-induced spurs, we propose a combination of the particle-based step-by-step (“SBS”) Brownian dynamics model and the compartment-based model (“SBS-RDME model”) for the simulation. We, finally, use the particle-based SBS Brownian dynamics model of Geant4-DNA as a reference to test the model implementation through several benchmarks. We find that the compartment-based model can efficiently simulate the system with a large number of species and for longer timescales, beyond the microsecond, with a reasonable computing time. Our aim in developing this model is to study the production and evolution of reactive oxygen species generated under irradiation with different dose rate conditions, such as in FLASH and conventional radiotherapy.

Highlights

The energy transfer induced by ionizing radiation in a water medium occurs rapidly (on a scale of femtoseconds) during the physical stage of water radiolysis and is followed by the formation of radiolytic species
We present the sub-volume method (NSM) [13], the choice of voxels size, and the merging technique using “the hierarchical algorithm for the reaction-diffusion master equation (RDME)” [14] approach in our implementation
We implemented, in Geant4-DNA, the compartment-based model combined with the SBS Brownian dynamics model already available in Geant4-DNA

Summary

Introduction

The energy transfer induced by ionizing radiation in a water medium occurs rapidly (on a scale of femtoseconds (fs)) during the physical stage of water radiolysis and is followed by the formation of radiolytic species These species are created in a very short time (from femtoseconds (fs) to picoseconds (ps)), mainly through electronic events during the physico-chemical stage [1,2,3,4]. Methods using Brownian dynamics and Smoluchowski theory have been applied to describe the spur evolution, in which detailed individual trajectories of spherical particles are simulated in chemical reaction-diffusion systems (called "systems" in the following text) [5,6,7,8,9] In this particle-based representation, only the molecules of interest are explicitly simulated, and the solvent (water) is considered a continuum. Their reactions continuously occur until they reach steady-states, where the species concentrations do not change anymore (from 100 microseconds (μs) to several seconds (s))

Methods

Results

Discussion

Conclusion