Breast cancer begins in the breast and spreads first to the lymph nodes of the armpit (axilla). When a breast lump is found to contain cancer, and if the cancer has not spread beyond the nodes of the axilla to distant sites, it is often removed surgically. Radiation therapy may be used in addition to surgery.

In certain cases of malignant lumps, lumpectomy followed by radiation therapy is as effective as a radical mastectomy. Typically, lumpectomy does not require a breast replacement (prosthesis).

In many cases of breast cancer, removal of the entire breast is unnecessary. A procedure such as a segmental mastectomy can be performed. In segmental mastectomy, the cancer and a larger area of normal breast tissue around it are removed. Lymph nodes under the arm and the lining over the chest muscles below the tumor also may be removed.

In some cases, the cancer is too large to be removed by lumpectomy. In these situations, removal of the breast along with the lymph nodes in the armpit (axilla) that drain the breast, is required. This procedure is called modified radical mastectomy (MRM). The results of mastectomy for breast cancer depend on the stage of the cancer, the tumor size, and whether there is cancer in the lymph nodes.

Chemotherapy is often administered after surgery for patients with cancer involving the lymph nodes.