The role of coating layers in gold nanorods' radioenhancement: a Monte Carlo analysis

Gold nanoparticles are promising radiosensitizing agents for nanoparticle-enhanced radiotherapy (NPRT). The coating layer on these nanoparticles can significantly influence their physicochemical characteristics and biological behavior. This study investigates the influence of various coating layers on the radioenhancement efficiency of gold nanorods by modeling the physical interactions and chemical reactions involved. We conducted Monte Carlo simulations using the TOPAS code to model secondary electron generation in gold nanorods exposed to 100 kVp X-rays. Through a multiscale approach, the dose contribution, electron spectrum, and <i>G</i>-values of radiolytic species were determined. Four conventional coating materials were examined and compared to a non-coated nanorod. The simulation results indicate that the addition of coating layers decreases the additional dose due to the gold nanorods by up to 7% across all materials. The assessment of electron spectra revealed that 1% to 8% of electrons with energies below 3.5 keV were absorbed within the various coating layers. In contrast, higher-energy electrons were mainly unaffected. The total <i>G</i>-values for all radiolytic species remained generally unchanged with the addition of the coating layer, regardless of the material used. However, increasing the coating thickness slightly increased the relative yield of chemical species at times beyond 10 ns post-irradiation. While the addition of a coating layer generally resulted in a decrease in electron fluence and dose contribution, the reduction was not as substantial as expected from results previously reported in the literature. This suggests that, from the physics perspective, the influence of coating layers on radioenhancement may be less pronounced than previously believed. Additionally, the observed increase in total <i>G</i>-values with thicker coatings emphasizes the need for further investigation into the effects of coatings on radiolytic species.