Dose Enhancement Due to Various Nanoparticles Irradiated by Megavoltage Photon Beams: A Review Study

Nowadays, the use of nanotechnology in cancer treatment offers exciting possibilities, including the enhanced delivery of doses inside the target, leading to a higher therapeutic ratio. The aim of this study is to review the dose enhancement factor (DEF) of various nanoparticles (NPs) in megavoltage radiotherapy. <i>In vitro</i>, Monte Carlo (MC) and experimental methods were employed in the reviewed studies, and the results indicated that higher DEF values were achieved through <i>in vitro</i> methods. According to the literature, smaller NPs tend to exhibit higher sensitization enhancement ratio (SER) in <i>in vitro</i> studies. In contrast, MC simulations report that NPs with higher concentrations, larger sizes, and higher atomic numbers led to greater DEF values. Smaller NPs penetrated tumors more effectively, whereas larger NPs had greater self-absorption of secondary electrons. It was also demonstrated that the increase in size may also result in higher toxicity and accumulation of NPs in tissues. A suitable balance between therapeutic efficacy, minimal toxicity, and uptake-related issues should be considered. Among the conducted studies, spherical and rod-shaped NPs have been more widely used. In addition, gold (Au) NPs in the size of 100 nm and concentration of 6 mM irradiating by 6 MeV photon beams have shown higher DEF values compared to other NPs in both MC and experimental methods (the maximum DEF reported was 4.71, representing a 53% increase for a 6 mM gel). Meanwhile, bismuth oxide (Bi₂O₃) NPs demonstrated the highest SER in <i>in vitro</i> studies, with a maximum SER of 7.64 for 90 nm particles at a concentration of 0.05 µM.