Inside a woman's breast are 15 to 20 sections called lobes. Each lobe is made of many smaller sections called lobules. Lobules have groups of tiny glands that can make milk. After a baby is born, a woman's breast milk flows from the lobules through thin tubes called ducts to the nipple. Fat and fibrous tissue fill the spaces between the lobules and ducts.
The breasts also contain lymph vessels. These vessels are connected to small, round masses of tissue called lymph nodes. Groups of lymph nodes are near the breast in the underarm (axilla), above the collarbone, and in the chest behind the breastbone.
Cancer begins in cells, the building blocks that make up tissues. Tissues make up the breasts and other parts of the body.
Normal cells grow and divide to form new cells as the body needs them. When normal cells grow old or get damaged, they die, and new cells take their place.
Sometimes, this process goes wrong. New cells form when the body doesn't need them, and old or damaged cells don't die as they should. The buildup of extra cells often forms a mass of tissue called a lump, growth, or tumor.
Tumors in the breast can be benign (not cancer) or malignant (cancer). Benign tumors are not as harmful as malignant tumors:
- are rarely a threat to life
- can be removed and usually don't grow back
- don't invade the tissues around them
- don't spread to other parts of the body
- may be a threat to life
- often can be removed but sometimes grow back
- can invade and damage nearby organs and tissues (such as the chest wall)
- can spread to other parts of the body
Breast cancer cells can spread by breaking away from the original tumor. They enter blood vessels or lymph vessels, which branch into all the tissues of the body. The cancer cells may be found in lymph nodes near the breast. The cancer cells may attach to other tissues and grow to form new tumors that may damage those tissues.