Because of its radio-biological and physical advantages, proton therapy method is highly popular compared to other radiotherapy methods. Recent attempts at enhancing dose in tumors and reducing dose in adjacent tissues have been mainly relying on the use of metal nanoparticles with the aim of activating tumors. Numerous qualitative studies have been carried out on the safety and clinical use of gold nanoparticles and the results have been promising. However, some quantitative studies should be

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... onducted to examine the factors affecting this method. With respect to the performance of Monte Carlo Method in simulation of the particles transport, the aim of this article was to obtain the dose enhancement in tumor sensitized by gold nanoparticles (GNPs) using MCNPX code. Methods: A slab head phantom with homogenized-GNPs-aided tumor was considered to simulate proton therapy in brain tissue. Monte Carlo simulation was performed using MCNPX code to assess the dose and its enhancement in a radio-sensitized tumor by GNPs using proton therapy. Findings: The Bragg peak calculations were conducted for proton beams with energies in the rage of 40-150 Mev. Dose and its enhancements in tumor were obtained for several concentrations. Then, Spread-out Bragg Peak (SOBP) was evaluated. Conclusion: According to our calculations, if the protons energy is high enough so that they can pass the sensitized tumor; dose will be improved in Bragg peak and then, will decrease immediately. In addition, in relation to the SOBP, solid dose enhancement in sensitized tumor and decreasing the dose immediately after it, were concluded. Dose enhancement due to the presence of nanoparticles in the tumor confirms the results of previous research. But, a significant reduction in the dose immediately after the tumor was the result of this research.