Dose Distribution of Electrons from Gold Nanoparticles by Proton Beam Irradiation

Purpose: In radiation therapy, gold nanoparticles (GNPs) are regarded as a promising radiosensitizer candidate. Several studies have revealed a dose enhancement by GNPs in X-ray and even proton irradiation. However, these studies have been limited to the depth direction. The dose distribution in both depth and lateral directions is crucial to evaluate the full radio sensitizing effect. The purpose of this study is to estimate the dose distribution around a GNP in terms of ejected electrons. Methods: The Geant4 Monte Carlo simulation toolkit was used to evaluate the energy deposition of electrons produced by a GNP. A 20 nm diameter spherical GNP was located in a water box and proton beams were incident unidirectionally. The energy deposition and location of produced electrons were tallied by 5 nm width water slabs at a variety of depths behind the GNP. The radial dose distribution was obtained in each slab. Results: The largest radial dose was observed in the slab closest to the GNP. At the slabs deeper than 90 nm, the dose in the radial direction within 10 nm from the beam direction was found to be smaller than that without GNP. This is because the presence of a GNP decreases the dose behind the GNP, forming a dose shadow. The dose enhancement both in depth and lateral directions was shown in surrounding areas. The area of distribution became larger as the absorbed dose decreased. Conclusion: The dose distribution around a GNP was estimated by a simulation study. The dose enhancement was observed in both the lateral and depth directions. This study will enable us to make use of GNPs as a radiosensitizer in proton therapy.