This study investigates the efficacy of gallium as a contrast agent for enhancing X-ray phototherapy in breast cancer tumors using the MCNPX code. A tumor was simulated within the breast section of the Medical Internal Radiation Dose (MIRD) phantom and irradiated with a 100 keV X-ray source. Gallium concentrations ranging from 1 % to 9 % (by weight) were introduced into the tumor volume to evaluate the enhancement of the deposited dose. Additionally, the reduction in scattered dose to nearby critical organs was assessed. The results indicate that increasing the gallium concentration significantly enhances the dose deposited in the tumor, reaching a saturation effect at higher percentages. Critically, the presence of 9 % gallium reduced the scattered dose to surrounding organs by a range of 62 %–93 %. This study demonstrates the potential of gallium-enhanced radiotherapy to simultaneously increase tumor dose while substantially protecting surrounding healthy tissues, thereby improving the therapeutic ratio.