Breast cancer is the most common cancer that occurs in women, with as many as one in eight women developing the disease during their lifetime. Breast cancer occurs when cells in breast tissue grow out of control, a situation called malignant.
The breast consists of about 15 to 20 sections called lobes, which have multiple smaller sections called lobules. These lobules end in tiny bulbs where milk is produced. Tiny tubes or ducts, leading to the nipple, connect the bulbs, lobules and lobes. Fibrous tissue and fat surround the lobes and all of this breast tissue rests on the muscle of the chest wall. The cells that line the ducts and lobules are the ones that become malignant.
Sometimes, cancer cells are found only inside the duct or lobule, called carcinoma in situ. In most cases, however, cancer cells have invaded through the wall of the duct or lobule, becoming “invasive cancer.” The cancer may be called invasive ductal cancer or invasive lobular cancer, identifying the cell type where it started. These cells may also invade into the breast’s lymph vessels and spread to the lymph nodes, primarily in the armpit. Or, cancer cells may invade the breast’s small blood vessels and travel to other body areas.