The effect of bismuth nanoparticles in kilovoltage and megavoltage radiation therapy using magnetic resonance imaging polymer gel dosimetry

Overcoming the tumor radioresistance along with protecting the adjacent normal tissues is considered as the ultimate goal of the radiation therapy. To achieve this goal, extensive studies have demonstrated the radio-sensitization potential of the gold nanoparticles (AuNPs) during treatment with low-energy ionizing radiation due to their high-photoelectric absorption coefficient. Compared to the AuNPs, the bismuth nanoparticles (BiNPs) with higher atomic number, low cost, high biocompatibility and high biodegradability are attractive candidates for amplifying the radiation dose. Accordingly, the aim of this study was to quantify dose enhancement factors (DEFs) from BiNPs compared to AuNPs at clinically relevant energies. To quantitatively monitor the three-dimensional distribution of the absorption dose, we synthesized the nPAG polymer gel impregnated with 0.2mM (0.04 mg/ml tissue) of AuNPs and BiNPs separately. In parallel, the samples were exposed to iridium-192 internal (380 keV) and cobalt-60 external (1.25 MeV) sources to explore the effect of energy. Finally, the R2-weighted dose maps of the calibration and test tubes were acquired by a Siemens 1.5 T MRI scanner. The mean dose enhancements obtained by AuNPs and BiNPs following irradiation of iridium-192 source were 14.72% +/- 0.34 and 16.35% +/- 0.38, respectively. Conversely, these values decreased to less than 4% for the samples irradiated with cobalt-60.-rays. Taken together, our experimental results suggest BiNPs-enhanced iridium-192 brachytherapy can potentially be a promising therapeutic tool in improving the efficacy of cancer radiation therapy. However, enhancement factors obtained from the cobalt-60 source decreased more significantly, highlighting the strong energy dependence of dose enhancement.