Breast cancer is a condition in which abnormal cell growth occurs in the breast. While it can affect men, the disease is more common in women. The causes of breast cancer are unknown, but family history of the disease and dense breast tissue are possible risk factors. Most women who are treated early for the condition resume healthy, active lives, although the risk of recurrence is present.
Women in the United States get breast cancer more than any other type of cancer, except for skin cancer. Each year, more than 200,000 American women learn they have the disease and over 40,000 women die from it.
Breast cancer is second only to lung cancer as a cause of cancer death in women. It occurs in men also, but the number of cases is smaller (see Male Breast Cancer).
Cancer is a group of more than 100 different diseases. It occurs when, for unknown reasons, cells become abnormal and divide without control or order. All parts of the body are made up of cells that normally divide to produce more cells, but only when the body needs them. With cancer, cells keep dividing, even when new cells are not needed.
The change from normal to cancerous cells requires several separate, different gene alterations. Eventually, altered genes and uncontrolled growth may produce a tumor that can be benign (not cancerous) or malignant (cancerous). Malignant tumors can invade, damage, and destroy nearby tissues and spread to other parts of the body. A benign tumor won't spread to other parts of the body, but local tissue may be damaged and the growth may need to be removed.
Cancer cells can also break away from a malignant tumor and enter the bloodstream or the lymphatic system. This is how the condition spreads within the body. When breast cancer spreads outside the breast, cancer cells often are found in the lymph nodes under the arm. They can also spread to other lymph nodes, the bones, liver, or lungs. Although it is not common, some people whose underarm lymph nodes are clear of breast cancer may still have cancer cells that have spread to other parts of the body.
Cancer that spreads to other parts of the body is the same disease and has the same name as the original cancer. For example, breast cancer that has spread to the bones is called metastatic breast cancer, not bone cancer.