Gold nanoparticles as a sensitising agent in external beam radiotherapy and brachytherapy: a feasibility study through Monte Carlo simulation

In recent years, gold nanoparticles (GNP) have been proposed as sensitising agents in radiotherapy. Several studies, performed both through Monte Carlo simulation of dosimetric and microdosimetric enhancement in presence of GNPs and through in-vitro experiments, have clearly shown the powerfulness of such a technique in radiotherapy. Aim of the present work is to estimate the dose enhancement during external beam radiotherapy and brachytherapy in tissues loaded with different GNP concentrations. We developed two Monte Carlo simulations in GEANT4: one reproducing a medical Linac operating at 6 MV, irradiating a volume of soft tissue containing GNP-doped sub-volumes at various depths; the second one representing an Ir-192 source irradiating a cylindrical cavity, in presence of a shallow volume distribution of GNPs. The Linac simulation yielded relative percent dose increases ranging from 1.6% for GNP concentrations of 1% in weight to 18% for 10% GNP concentrations, while the high-energy brachytherapy Ir-192 source gave 6% of relative dose increase at 1% of concentration up to more than 70% increase at 10% of GNP concentration. Such dose enhancements are mainly due to the increased photoelectron and characteristic X-ray production from the GNPs and, at a lower percentage, to the Auger electrons escaping from the GNPs. These scarcely penetrating radiations induce relevant microdosimetric intensifications in the surrounding of each GNP, leading to a biological damage at cellular and sub-cellular level, which depends on the biological distribution of GNPs within the target cells.