Impact of self-absorption on dose enhancement induced by metallic nanoparticles

Metallic nanoparticles (NPs) have significant potential to enhance radiotherapy by increasing tumor cell damage while sparing healthy tissues. This radiosensitization effect primarily results from the generation of secondary electrons when radiation interacts with NPs and is quantified by the dose enhancement factor (DEF). However, DEF is influenced by the self-absorption effect, which depends on NP size, composition, and the incident radiation energy. In this study, we used the Monte Carlo simulation code PENELOPE (version 2014) to quantify self-absorption effects in various NP configurations and radiation beams. Simulations were conducted by varying NP material (gold, silver, platinum, and bismuth), size, and concentration, while exposing them to X-rays at 100 kVp and 6 MV, as well as gamma rays from¹⁹²Ir and⁶⁰Co sources. The resulting secondary electron spectra were analyzed to determine key parameters, including average energy, range, and DEF. Results indicate that for all analyzed beams and materials, both the energy and average range of secondary electrons increase with NP size. DEF is maximized for low-energy beams and for materials with high atomic numbers and concentrations, as well as for the smallest NPs due to self-absorption. At 100 kVp and 1% NP concentration of gold for example, DEF decreased by 9.5% when NP size increased from 2 nm to 100 nm, whereas for higher-energy beams (6 MV and⁶⁰Co), this reduction was below 0.1% across all materials and concentrations. These findings reinforce the potential of NPs in radiotherapy and highlight the impact of self-absorption on NP dosimetry, affecting mean energy, range, and DEF. Notably, self-absorption is most pronounced for lowenergy beams - the same conditions where DEF is maximized.&#xD.