Mastectomy is the medical term for the surgical removal of one or both breasts, partially or completely. A mastectomy is usually carried out to treat breast cancer. In some cases, women believed to be at high risk of breast cancer have the operation as a preventive measure. Alternatively, some women can choose to have a wide local excision, also known as a lumpectomy, an operation in which a small volume of breast tissue containing the tumor and a surrounding margin of healthy tissue is removed to conserve the breast. Both mastectomy and lumpectomy are referred to as "local therapies" for breast cancer, targeting the area of the tumor, as opposed to systemic therapies, such as chemotherapy, hormonal therapy, or immunotherapy.

The decision to perform a mastectomy is based on various factors, including breast size, the number of lesions, biologic aggressiveness of a breast cancer, the availability of adjuvant radiation, and the willingness of the patient to accept higher rates of tumor recurrences after lumpectomy and/or radiation. Outcome studies comparing mastectomy to lumpectomy with radiation have suggested that routine radical mastectomy surgeries will not always prevent later distant secondary tumors arising from micro-metastases prior to discovery, diagnosis, and operation. In most circumstances, there is no difference in both overall survival and breast cancer recurrence rate. While there are both medical and non-medical indications for mastectomy, the clinical guidelines and patient expectations for before and after surgery remain the same.