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. In vitro, Monte Carlo (MC) and experimental methods were employed in the reviewed studies, and the results indicated that higher DEF values were achieved through in vitro methods. According to the literature, smaller NPs tend to exhibit higher sensitization enhancement ratio (SER) in in vitro 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 in vitro studies, with a maximum SER of 7.64 for 90 nm particles at a concentration of 0.05 µM.