The breast is composed of 15 to 20 sections called lobes. Within each lobe are many smaller lobules. Lobules end in dozens of tiny bulbs that can produce milk. The lobes, lobules, and bulbs are all linked by thin tubes called ducts. These ducts lead to the nipple in the center of a dark area of skin called the areola. Fat surrounds the lobules and ducts. There are no muscles in the breast, but muscles lie under each breast and cover the ribs.

Each breast also contains blood vessels and lymph vessels. The lymph vessels carry colorless fluid called lymph, and lead to small bean-shaped organs called lymph nodes. Clusters of lymph nodes are found near the breast in the axilla (under the arm), above the collarbone, and in the chest. Lymph nodes are also found in many other parts of the body.

Breast cancer occurs when the cells in the breast begin to grow and replicate in an abnormal and uncontrolled way. When this happens, the body cannot organize these cells for normal function and the cells form a mass that is called a tumor. Malignant tumors in the breast can spread to other parts of the body, crowding and destroying normal cells. Breast cancer often grows very slowly over a period of years.

All women (and even men) are potentially at risk of developing breast cancer at some point in their lifetime. The most common risk factors for breast cancer include

• age over 50;
• a history of prior breast cancer;
• a family history of breast cancer;
• certain changes in the breast;
• certain genetic alterations;
• extended exposure to estrogen;
• late childbearing;
• breast density;
• radiation therapy; and
• alcohol

The most common symptoms of breast cancer include

• a lump or thickening in or near the breast or in the underarm area;
• a change in the size or shape of the breast;
• nipple discharge or tenderness, or the nipple pulled back (inverted) into the breast;
• ridges or pitting of the breast (the skin looks like the skin of an orange); and
• a change in the way the skin of the breast, areola, or nipple looks or feels (for example, warm, swollen, red, or scaly).