Where does breast cancer start?

Breast cancer starts in the breast, and it can begin in one or both breasts. It happens when cells in the breast grow without control and form a mass called a tumor.

Breast cancer can form in several kinds of breast tissue:

• Glandular tissue — the milk ducts and the lobules (the tiny glands that make milk). Cancers that start in the ducts are called ductal cancers, and cancers that start in the lobules are called lobular cancers. Most breast cancers start in the ducts.
• Fibrous and fatty tissue (stroma) — the tissue that gives the breast its shape.
• The nipple — where a cancer called Paget disease of the breast can begin.
• Blood vessels and lymph vessels — where inflammatory breast cancer and angiosarcoma can begin.

Your healthcare team can explain where your breast cancer started and what that means for you.