The system INTRABEAM ™ is mainly for breast cancer treatment for tumors less than or equal to 2 cm in diameter. Spherical applicators with varying diameters depending on the tumor size (2.5 cm to 5 cm) are mounted on the system to have an isotropic distribution of the dose. After resection of the tumor, the surgeon inserts the applicator in place of the tumor and the system delivered a single fraction of 20 Gy to the surface of the applicator for irradiating the tumor bed with a beam of X-rays low-energy (50 kV). There is no requirement customized and this is questionable since an official document recommends a single dose optimization. This study is divided into two stages:(1) A model of the INTRABEAM™ system was created realistically and simple simulations in a water phantom were performed to validate the model.(2) In the second part, the model is used to perform dose calculations customized from CT images of the patient, to take into account tissues heterogeneities. Monte Carlo simulations on the GATE platform are made precisely as the experimental geometry. The depth dose curves for the bare probe and for applicators available are analyzed. The experimental dose measurements were performed with an ionization chamber PTW and thermoluminescent dosimeters. The isotropy of the source has been also verified. The simulation results are compared with experimental measurements.