Cancer is a group of more than 100 different diseases. Cancer 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 only when the body needs them. When cancer occurs, 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 cancer) or malignant (cancer). 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.

A malignant tumor can invade surrounding tissue and destroy it. Cancer cells can also break away from a malignant tumor and enter the bloodstream or the lymphatic system. This is how cancer spreads within the body. When breast cancer spreads outside the breast, cancer cells often are found in the lymph nodes under the arm. Cancer cells may spread beyond the breast such as to other lymph nodes, the bones, liver, or lungs. (Although it is not common, some patients whose underarm lymph nodes are clear of breast cancer may still have cancer cells which 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. When breast cancer spreads, it is called metastatic breast cancer even though it is found in another part of the body. For example, breast cancer that has spread to the bones is called metastatic breast cancer, not bone cancer.