Breast cancer develops from the uncontrolled growth of abnormal cells in the breast tissue. These growing cells form a tumor in the breast, which can sometimes be felt as a lump. There are several types of breast cancer, but the most common type is ductal carcinoma, which arises in the milk ducts (the passages through which milk flows toward the nipple). A small percentage of breast cancers develop in the lobules (small glands in which milk is produced) and are called lobular carcinoma.
Ductal and lobular cancers are further classified by whether the cancer cells have spread to other parts of the body. If the breast cancer cells are confined within the milk ducts or lobules, it is called noninvasive or in situ cancer. When the cancer has grown into surrounding tissue or spread to other parts of the body, it is called invasive or infiltrating cancer. About 80 percent of all breast cancers are invasive ductal carcinomas (IDC).
Why do these cells start growing abnormally? We'll investigate on the next page.