The breasts sit on the chest muscles that cover the ribs. Each breast is made of 15 to 20 lobes. Lobes contain many smaller lobules. Lobules contain groups of tiny glands that can produce milk. Milk flows from the lobules through thin tubes called ducts to the nipple. The nipple is in the center of a dark area of skin called the areola. Fat fills the spaces between the lobules and ducts.
Breast cancer incidence is much higher in industrialised Western countries, whether in Europe or North America, than in developing countries. North American women have the highest incidence of breast cancer in the world. Among women in the U.S., breast cancer is the most common cancer and the second-most common cause of cancer death (after lung cancer). Women in the U.S. have a 1 in 8 (12.5%) lifetime chance of developing invasive breast cancer and a 1 in 35 (3%) chance of breast cancer causing their death. In 2007, breast cancer was expected to cause 40,910 deaths in the U.S. (7% of cancer deaths; almost 2% of all deaths)