Background: External beam radiation therapy is often administered to patients implanted with silicone gel breast prosthesis. The aim of this study was to investigate the influence of silicone gel breast prosthesis on photon dose distributions. In the event of recurrence, the oncologist may be forced to irradiate through the prosthetic device. To quantify the dose enhancement or reduction below the silicone gel breast prosthesis, depth dose enhancement factors (DEFs) were calculated. Materials and Methods: The study was based on Varian linear accelerator (LINAC) operated at 6 and 15 MV photon energies. Monte Carlo package Electron Gamma Shower (EGSnrc) was employed to simulate the depth dose distribution in a three dimensional scanning water phantom with various field sizes. The polydimethyl silicone gel breast prosthesis with density of 0.97 g/cm³ was used. The measured and calculated DEFs were verified by using the thermoluminescent dosimeter (TLD). Results: The results indicate that the percentage difference between the calculated and measured dose distributions on depth dose curves for 6 and 15 photon energies was less the 2% for all locations. DEFs at 0.5 cm below the 3.5 cm prosthesis were 0.99 and 1.02 for 6 and 15 MV photon beams, respectively. The interface region receives enhanced dose of about 2.4% with 15 MV photon beam while the 6 MV photon beam delivered a dose reduction of about 2.0%. Conclusion: It was observed that DEFs increase with photon beam energy. The 6 MV photon beam reduces dose enhancement factor compared to that of the 15 MV photon beam.