We present a model based on patient anatomy to predict the percentage of ipsilateral lung and/or heart volume receiving at least a certain dose in breast treatments with static tangential fields of external radiotherapy. The introduction will briefly guide us through the disease of breast cancer and the importance of external radiotherapy in its treatment, as well as the work involved in a radiotherapy treatment until we reach the core of the work: the theoretical framework of the model. The variables defined in the theoretical framework and computable through the DICOM structure file of each patient, will be useful to predict the volume occupied by the isodose within the organs at risk.