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.
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 usually not harmful
- Rarely invade the tissues around them
- Don't spread to other parts of the body
- Can be removed and usually don't grow back
- May be a threat to life
- Can invade nearby organs and tissues (such as the chest wall)
- Can spread to other parts of the body
- Often can be removed but sometimes grow back
Breast cancer cells can spread by breaking away from a breast tumor. They can travel through blood vessels or lymph vessels to reach other parts of the body. After spreading, cancer cells may attach to other tissues and grow to form new tumors that may damage those tissues.
For example, breast cancer cells may spread first to nearby lymph nodes. Groups of lymph nodes are near the breast under the arm (axilla), above the collarbone, and in the chest behind the breastbone.
When breast cancer spreads from its original place to another part of the body, the new tumor has the same kind of abnormal cells and the same name as the primary (original) tumor. For example, if breast cancer spreads to a lung, the cancer cells in the lung are actually breast cancer cells. The disease is metastatic breast cancer, not lung cancer. For that reason, it's treated as breast cancer, not lung cancer.
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