What Is Breast Cancer?

Illustration of the female breast anatomy. On the left, a front view shows lymph nodes inside the breast going from the breast to the armpit. On the right, a cross-section shows the chest wall, ribs, fatty tissue, lobes, ducts, and lobules. Also shown in both panels are the muscle, nipple, and areola. — The female breast contains lobes, lobules, and ducts that produce and transport milk to the nipple. Fatty tissue gives the breast its shape, while muscles and the chest wall provide support. The lymphatic system, including lymph nodes, filter lymph and store white blood cells that help fight infection and disease.

Credit: © Terese Winslow

Breast cancer is cancer that starts in the breast. It can start in one or both breasts. Breast cancer happens when cells in the breast grow without control, creating a mass called a tumor that may spread elsewhere in the body. Breast cancer mostly affects females aged 45 and older, but anyone with breasts can get breast cancer. It is rare in children and males.

Breast cancer can form in:

Glandular tissue. This tissue includes milk glands, milk ducts, and lobules. Lobules are tiny glands that make milk. After a baby is born, breast milk flows from the lobules through thin tubes (ducts) to the nipple. Cancers that start in the ducts are called ductal cancers. Cancers that start in the lobules are called lobular cancers. Most breast cancers are ductal cancers.
Fibrous tissue and fatty tissue (stroma). These tissues fill the spaces between the lobules and ducts and hold breast tissue in place. They give breasts their shape and size. A type of breast tumor called phyllodes tumor can start in the stroma.
The nipple. This is the small, raised area in the center of the breast that contains duct openings through which milk can leave the breast. A type of breast cancer called Paget disease of the breast can start in the nipple.
Blood vessels and lymph vessels. Inflammatory breast cancer occurs when cancer cells block lymph vessels in the skin of the breast. Angiosarcoma can start in the cells that line blood vessels and lymph vessels in the breast.

There are many types of breast cancer, depending on where it begins and the extent of spread. When the abnormal cells are within the lobules or ducts and have not spread to other tissues in the breast, it is called carcinoma in situ. Invasive cancers have spread into surrounding breast tissue and can spread to nearby lymph nodes or other organs throughout the body. Most breast cancers are invasive.