Breast cancer is a malignant (cancerous) tumor that starts from cells of the breast. The disease occurs primarily in women, but men can get breast cancer as well. The information here refers only to breast cancer in women, but separate information about breast cancer in men can be found on our Web site.
A woman's breast is made up of glands that make breast milk (lobules), ducts (small tubes that connect lobules to the nipple), fatty and connective tissue, blood vessels, and lymph (pronounced limf) vessels. Most breast cancers begin in the ducts (ductal carcinoma), some begin in the lobules (lobular carcinoma), and the rest in other tissues.
Lymph vessels are like veins, except that they carry lymph fluid instead of blood. Lymph is a clear fluid that contains immune system cells and waste products. Lymph vessels lead to small, bean-shaped collections of tissue called lymph nodes. Most lymph vessels of the breast lead to lymph nodes under the arm. These are called axillary (AX-uh-lair-ee) nodes. If breast cancer cells reach the underarm lymph nodes and continue to grow, they cause the nodes to swell. Once cancer cells have reached these nodes, they are more likely to spread to other organs of the body.
Aside from being female, age is the most important factor affecting breast cancer risk. Moderate or vigorous physical activity, maintaining a healthy body weight, having first child before age 30, breastfeeding, and avoiding alcohol or drinking less than 2 drinks each day are all associated with a lower risk of breast cancer.
To learn more about breast cancer and how you can reduce your risk or find it early so you can stay well, please visit cancer.org/breastcancer or call us toll-free at 1-800-227-2345 anytime, day or night.