Recent updates of the MPEXS2.1-DNA Monte Carlo code for simulations of water radiolysis under ion irradiation

Abstract

To improve radiotherapy, especially that with ion beams such as proton and carbon ion beams, the mechanisms of interactions induced by ionizing radiation must be understood. MPEXS2.1-DNA is a Monte Carlo simulation code developed for water radiolysis studies and DNA damage simulations that uses GPU devices for fast computation. However, the original chemistry model in MPEXS2.1-DNA did not include detailed chemical reactions for reactive oxygen species (ROS), e.g., O^•-, O₂, O₂^•-, HO₂^•, HO₂^-. In the present study, drawing the former work on the step-by-step (SBS) model for the RITRACKS code, we implemented an alternative SBS model into MPEXS2.1-DNA to increase the capabilities and computational speed of water radiolysis simulations under ion irradiation. This model is based on the theory of Green's function of the diffusion equation (GFDE-SBS). Also, we implemented multiple ionization processes which enhance ROS generation under high-LET irradiation. We compared the simulation results obtained by GFDE-SBS with experimental data from previous studies. The validation results demonstrated that the GFDE-SBS model accurately reproduced the measured radiation chemical yields of major species, such as hydroxyl radicals and hydrogen peroxide. Furthermore, the computational speed of GFDE-SBS was increased approximately ten times faster than the original model due to the changes in time stepping. Additionally, simulations using a Fricke dosimeter confirmed that this model is reliable for long-term simulations over seconds. These improvements enable simulations of radiation interactions and can help in the study of DNA damage mechanisms.

Keywords: GPGPU; Monte Carlo simulation; Radiation chemistry; Radiation physics; Water radiolysis simulation.

Fig. 1

Time dependence of the G values of (a) hydroxyl radicals, (b) hydrated electrons, (c) hydrogen peroxide, (d) hydrogen molecules, and (e) OH^- anions under 750 keV electron irradiation. The solid and dashed lines are the simulation results obtained using the GFDE-SBS and CONV-SBS approaches, respectively. The symbols represent theoretical calculation^– and experimental^– data. The shade region in each panel shows the standard deviation of the calculated G value for each species. The standard deviation of hydroxyl radicals and hydrated electrons are less than 1%; thus, they are invisible.

Fig. 2

Time evolution of molecular species, simulated by the GFDE-SBS model, after irradiation by carbon ions with a kinetic energy of 5 MeV/u at (a) t = 2.0 x 10^–6 µs, (b) 0.05 µs, (c) 0.10 µs, (d) 0.50 µs, and (e) 0.94 µs. Change of color is due to chemical reactions.

Fig. 3

LET dependence of the chemical yields of (a) hydrated electrons, (b) hydroxyl radicals, (c) hydrogen radicals, and (d) hydrogen molecules under irradiation with protons (¹H⁺), alpha particles (⁴He²⁺), and carbon ions (¹²C⁶⁺). Our simulation results obtained by GFDE-SBS for each ion are shown as solid and dashed lines of different colors. The solid lines represent the chemical yields calculated at one microsecond, whereas the dashed lines represent those calculated at ten nanoseconds after irradiation. The symbols represent experimental data^–,–. The shaded regions represent a ± 2% range of the standard deviation of G values.

Fig. 4

LET dependence of the chemical yield for hydrogen peroxide, G(H₂O₂), under irradiation with (a) protons, (b) alpha particles, and (c) carbon ions. The blue solid lines are our simulation results obtained by GFDE-SBS. The magenta solid line represents the reevaluation of G(H₂O₂) for ¹²C⁶⁺ as the “track-averaged G value.” The symbols represent experimental data from various studies ^–,–. The standard deviation of G(H₂O₂) is less than 1% for each case; therefore, a ± 2% range is represented for better visual recognition.

Fig. 5

The occurrence of chemical reactions, shown as radiation chemical yields, under carbon ion irradiation with an incident energy of 1, 2, 4, and 8 MeV/u; (a) is the generation of hydrogen peroxides by R1, while (b) and (c) are the consumption of hydrogen peroxides by R2 and R3, respectively.

Fig. 6

(a) Chemical yield of Fe³⁺ as a function of time calculated by GFDE-SBS for irradiation with 100 MeV protons up to 100 s (red solid line). The filled circle with a vertical error bar represents the reported chemical yield of Fe³⁺. (b) LET dependence of G(Fe³⁺). The solid red line represents the simulation results, calculated at 100 s after irradiation with protons of various incident energies. The symbols represent measured data from various studies^–. A ± 2% range of the standard deviation of G(Fe³⁺) is displayed as a shaded region.