Breast cancer is a disease in which cancer cells grow in the breast. Normally, the cells of the breast divide in a regulated manner. If cells keep dividing when new cells are not needed, a mass of tissue forms. This mass is called a tumor. A tumor can be benign or malignant.

Breast Cancer
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A benign tumor is not cancer and will not spread to other parts of the body. A malignant tumor is cancer. Cancer cells divide and damage tissue around them. They can enter the bloodstream and spread to other parts of the body. This can be life-threatening.

Breast cancer is the most common form of cancer found in women; it is estimated that over 182,000 women were diagnosed with breast cancer in the US in 2008. Although the majority of breast cancer cases occur in women, men can develop it as well; approximately 2,000 men developed breast cancer in the US in 2008.

Caucasian, Hawaiian, and African-American women have the highest rates of breast cancer in the US. The lowest rates occur among Korean, Native American, and Vietnamese women.