Cancers are a group of diseases that cause cells in the body to change and grow out of control. Most types of cancer cells form a lump or mass called a tumor and are named after the part of the body where the tumor originates.

Breast cancer begins in breast tissue, which is made up of glands for milk production, called lobules, and the ducts that connect lobules to the nipple. The remainder of the breast is made up of fatty, connective, and lymphatic tissue.

•Most breast abnormalities are benign; that is, they are not cancerous, do not grow uncontrollably or spread, and are not life-threatening.

•Some breast cancers are called in situ, because they have not yet spread beyond the area where they began. In situ breast cancers are confined within the ducts (ductal carcinoma in situ) or lobules (lobular carcinoma in situ) of the breast. Nearly all cancers at this stage can be cured. Many oncologists believe that lobular carcinoma in situ (also known as lobular neoplasia) is not a true cancer but rather is an indicator of increased risk for developing invasive cancer in the future.

•Other cancerous breast tumors are invasive, or infiltrating. These cancers start in the lobules or ducts of the breast but have broken through the duct or gland walls to invade the surrounding fatty tissue of the breast.