Breast cancer is a malignant tumor in the glandular tissues of the breast. These tumors (called carcinomas) form when normal cell growth becomes uncontrolled, enabling a single abnormal cell to multiply at a rapid rate. Carcinomas tend to destroy an increasing proportion of normal breast tissue over time, and may spread, or metastasize, to other sites.

Breast cancer is the most common cancer in women, other than skin cancer. Breast cancer is especially prevalent in developed countries. As a result of large-scale screening of women considered at higher risk, a greater number of breast cancers have been discovered and treated in recent years, even in cases where the women experienced no symptoms.

It is estimated that 203,500 new cases are diagnosed and about 40,000 women die each year from cancer originating in the breast. One in 8 American women who live to age 85 will develop this illness at some time during her life. Risk increases with age, and women 75 years and older are at highest risk.

Breast cancer can affect males, but the disease strikes women about 100 times as often as it does men. Some of the risk factors for breast cancer include genetics, diet, age at menstruation, and prolonged exposure to female sex hormones (estrogen).

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