Before discussing breast cancer, it's important to be familiar with the anatomy of the breast. The normal breast consists of milk-producing glands that are connected to the surface of the skin at the nipple by narrow ducts. The glands and ducts are supported by connective tissue made up of fat and fibrous material. Blood vessels, nerves, and lymphatic channels to the lymph nodes make up most of the rest of the breast tissue. This breast anatomy sits under the skin and on top of the chest muscles.
As in all forms of cancer, the abnormal tissue that makes up breast cancer is the patient's own cells that have multiplied uncontrollably. Those cells may also travel to locations in the body where they are not normally found. When that happens, the cancer is called metastatic.