Chemical mechanism in gold nanoparticles radiosensitization: A Monte Carlo simulation study

Abstract

Gold nanoparticles (AuNPs) can enhance radiotherapy efficacy via physical and chemical mechanisms, with their individual contributions varying depending on the type of radiation. Moreover, AuNP-enhanced radiotherapy efficacy depends also on nanoparticle size and number, and on a complex dynamics of reactive oxygen species (ROS) production. Monte Carlo simulations of irradiating a single nanoparticle with Co-60 $\gamma$-rays, 160 kVp X-rays, 14 MeV and 200 MeV proton beam were performed to estimate the enhancement of ROS production. The enhancement was also evaluated as a function of the NP diameter. The simulation setup was such that lateral charged particle equilibrium (CPE) was achieved, and the results were normalized to the fluence of the incident radiation field. The findings confirm previous reports that ROS enhancement is greater for low LET protons compared to high LET protons and for low-energy X-rays compared to Co-60 $\gamma$-rays. However, the enhancement due to a single AuNP is found to be orders of magnitude smaller if CPE conditions prevail. This statement also holds for the estimated enhancement for multiple AuNPs at mass fractions in the percent level. The dependence of ROS enhancement on NP size varies with the type of radiation, following distinct trends: an almost cubic relationship for low-energy X-rays, a quadratic relationship for Co-60 and 200 MeV protons, and an intermediate relationship for 14 MeV protons. These trends correspond closely to the yield of secondary electrons emitted by the NPs, which drive ROS production. This study provides insights into the mechanisms of ROS enhancement, contributing to the optimization of nanoparticle-enhanced radiation therapy.