Cancer is a group of diseases that cause cells in the body to change and grow out of control.  Cancer can start any place in the body.  It can start in the breast, the lungs, the colon or even in the blood.  Cancers that starts in the breast is called breast cancer.  Cancer cells can spread to other parts of the body.  For instance, cancer cells in the breast can spread to the bone and grow there.  When cancer cells spread, it is called metastasis.  However cancer is always named for the place where it starts.  So when breast cancer spreads to the bone, it is still called breast cancer.diagram of the lymph system

Most types of cancer cells eventually 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 masses or lumps are benign; that is, they are not cancerous, do not grow uncontrollably or spread, and are not life-threatening. Some breast cancers are called insitu because they 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 a cancer, but an indicator of increased risk for developing invasive cancer in either breast.

Most malignant or 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 glandular walls to invade the surrounding tissue of the breast.

Last Medical Review: August 2010