The MCNPX code was used in this work for the simulation of a treatment for breast cancer, therefore a geometry of a CT phantom (FAX06) was built through a software ImageJ which allows us to import DICOM images and then convert them into valid input files for the code. A simplified geometry of a linear accelerator (LINAC) bolster was then built, it is composed of a spherical shielding of Pb and W, and a MLC system, equivalent to those used on a VARIAN 2300 C/D. 60Co, radiation beams were emitted from the center of the shielding 6 and 15 MV spectrum with an angulation to recreate the two tangential fields used in conventional treatments of breast cancer radiotherapy. The main objective of this work, was to estimate the absorbed dose in each one of the organs involved in this type of treatments. The calculation was made on a parallelized cluster, taking as reference a planification file of a conventional breast cancer treatment from a VARIAN eclipse system. The outcome permitted to establish the differences between the absorbed dose of organs in risk that are submitted like; lungs and heart, especially when different types of radiation beams are used, allowing us to establish which radiation beam is more convenient from the point of view of radiology protection to organs in risk for this treatment. The most important conclusion lies in the dose that is administered with Co–60 specifically in breast treatments. The almost monoenergetic energy of 1.17 MeV presents an interesting distribution with good coverage and the advantage of minimizing doses to organs at risk.