Calculation of X-ray dose enhancement factor in assumed labeled tumor by Gd contrast agent using Geant4

In order to increase the absorbed dose in the tumorous area, photon activation therapy via labeling of tumor by heavy contrast agents such as gadolinium (Gd) and target exposing by ortho-voltage x-rays is used. In this method, the photoelectric effect is dominant in the tumorous area which will lead to the increase of local dose. By using Geant4 computer code, dose distribution in different areas of normal tissue and Gd element tumor-activated, were obtained. In the designed model, the x-ray source was considered in the shape of a mono energetic and superficial circular plate with the radius of 15mm and the phantom (normal tissue, assumed tumor and detectors) in cubic shape with the side of 13cm. Rectangular cubic shape detector area dimensions’ are 40×40×48 mm3 which continue from phantom surface to a depth of 48.67 mm along x, y and z axes. Optimum energy of photons in order to maximum absorbed dose enhancement factor (DEF) in gadolinium smeared tumorous area is 59.6 keV. By increasing the concentration of contrast agents, the homogeneity of dose distribution in assumed tumorous area is decreased but the greatest homogeneity of dose is seen at 106.5 keV of energy. Increased fluctuations of absorbed dose relative to the contrast agents’ concentration indicate that these changes in the concentration range of 1-8 mg/ml follow the linear function. Based on the simulation results in this study, absorbed dose in tumorous area following addition of contrast agent (Gd) with specifying the concentration and photon energy, is significantly more than when the assumed tumor is not labeled.