The effects of gold nanoparticles concentrations and beam quality/LET on dose enhancement when irradiated with X-rays and protons using alanine/EPR dosimetry

Abstract This research investigates the effects on dose enhancement (DE) caused by secondary electrons when varying concentrations of gold nanoparticles (AuNPs) are irradiated with different quality beams (proton, kV and MV X-rays) over clinical doses ranges (2–20 Gy). Spherical gold-nanoparticles (5 nm) were added to l -alanine, producing composites of 3, 2 and 1% (w/w) AuNPs, dispersed uniformly within wax pellets of dimensions 10 × 5 × 5 mm. The pellets were irradiated within a phantom alongside control pellets ( l -alanine) and the yield of alanine radicals quantified by Electron Paramagnetic Resonance (EPR) spectroscopy. Comparison of the EPR signal produced by the AuNP/alanine composites to that of the control l -alanine pellets is reported as the DE. For X-rays (kV and MV) the DE increased with increasing AuNP concentration, with kV X-rays (80 kV) having the highest levels of DE; 55% for the 3% AuNP/alanine composites, which reduced to 15% for the 1 %-composites. Whilst 6 MV X-rays showed a DE of 17% for the 3 %-composites, which was reduced to 2% DE for the 1 %-composites. Irradiations with protons (150 MeV, SOBP) showed no such dependence on the AuNP concentration and consistently yielded DE levels of ≤5% for all AuNP concentrations investigated. These results agree with recent Monte Carlo simulations reporting little to no secondary electron production when AuNPs are irradiated with protons, and supports studies that the higher DE seen in cell and animal studies (ca.15–20%) for AuNPs irradiated with protons is due to other processes, such as the production of reactive oxygen species (ROS) generated from the aqueous media in cells, which are not measured in this experiment.