Breast cancer is the most common type of cancer in women, and the second most common cancer in North America. Every year 183,000 new cases of breast cancer are diagnosed in the United States, and 19,200 cases are diagnosed in Canada. The majority of cases occur in women over 50, and as with most cancers, the risk of developing breast cancer increases with age. An American woman who lives to age 80 has a one in eight chance of developing breast cancer. Breast cancer strikes men as well as women, but male breast cancer accounts for less than 1 percent of all breast cancer cases. Cancer researchers have found that approximately 5 percent of all breast cancer cases are associated with inherited mutations in genes such as BRCA1, BRCA2, and p53. The protein HER-2 (also called HER2/neu), produced by oncogenes, is present in about one-third of all breast cancers. Other risk factors include a family history of the disease, early onset of menstruation, late menopause, never having children, or having a first child after age 30, conditions that seem to extend the duration of estrogen exposure in the body.

The breast is made up of milk-secreting glands called lobules; ducts that connect the glands to the nipple; and fatty, connective, and lymphatic tissue. Breast cancer occurs in any of these components of the breast. Ductal carcinomas account for 80 percent of all breast cancers. Between 10 and 15 percent of breast cancers are lobular carcinomas. Other types of breast cancer are much more rare; combined, they make up the remaining 5 to 10 percent of all breast cancer cases.

Most breast cancers are first detected as an unusual mass or lump in the breast. If the cancer is detected and treated early, the odds of recovering from breast cancer are quite good. The American five-year survival rate for all breast cancers is 83 percent, but if the tumor is detected early, it rises to 96 percent.