Monte Carlo model for evaluation of concentration of gold nanoparticle clusters as predictor of effective dose in proton therapy of microscopic tumors

Radio-sensitization with gold nanoparticles (GNPs) is a relatively recent advancement of radiotherapy that is determined to increase radiation in the target tumor region. Monte Carlo (MC) simulation is a well-known method that has gained popularity by assisting in determining the effective dose in various tumors. Assessing the concentration of particles during proton therapy can provide the key to evaluating the efficiency of the performed treatment. Therefore, this study aimed to investigate the interconnection between the concentration of gold nanoparticle clusters and the dose enhancement in microscopic tumors by using the MC Model. A single cell tumor model with a size of 5 μm that was exposed to 100 MeV protons in a region resembling the human brain was investigated by using an MC simulation employing the Geant4 toolkit. With each nanoparticle having a diameter of 15 nm, a three-dimensional model of the random distribution of gold nanoparticle clusters was developed. Then, the dose enhancement for various concentrations of the GNPs accumulated around the tumor was estimated according to the dose enhancement factor (DEF), while the dose was concentrated on the surface of each GNP in the clusters. The results showed that the dose enhancement located at the Bragg-peak in the tumor region depended on the different concentrations of the GNPs. DEF values were found to be 1.016, 1.022, 1.026, 1.035, 1.013, 1.000, and 0.984 for the 1, 2, 3, 5, 10, 30, and 50 mg/ml concentration levels in water, respectively. The findings also revealed that large GNP clusters with high concentration levels yield larger DEFs than clusters with lower concentration levels, which may have an impact on radiation therapy. Specifically, at a concentration higher than 10 mg/ml, the estimated dose was significantly lower than those for a concentration of 5 mg/ml by about ∼100 mGy. As the benefit of the increased total dose must be balanced against the effect of the accumulation of nanoparticles, the current study detected dose reduction in regions that accumulate GNPs at high concentrations. As a result, the potential efficiency of the radiotherapeutic treatment can be reduced.