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.

The breast consists of lobes, lobules, and bulbs that are connected by ducts. The breast also contains blood and lymph vessels. These lymph vessels lead to structures that are called lymph nodes.

Clusters of lymph nodes are found under the arm, above the collarbone, in the chest, and in other parts of the body. Together, the lymph vessels and lymph nodes make up the lymphatic system, which circulates a fluid called lymph throughout the body. Lymph contains cells that help fight infection and disease.

When breast cancer spreads outside the breast, cancer cells are most often found under the arm in the lymph nodes. In many cases, if the cancer has reached the lymph nodes, cancer cells may have also spread to other parts of the body via the lymphatic system or through the bloodstream.