Breast cancer is understood by most people as a disease limited to the breast. However, breast cancer can spread or metastasize to nearby lymph nodes, usually those under the arm. That is why surgery for breast cancer always involves some type of surgery for the glands under the arm. Sometimes a sentinel node biopsy is performed to test the glandular tissue for the presence of cancer cells.

Breast cancer can also spread to other parts of the body via blood vessels, and the term metastatic breast cancer indicates that the cancer has spread from the breast and beyond the lymph nodes to other parts of the body such as bone, lung, liver, or brain. A secondary cancer tumor will form in these distant sites as the cancer cells collect in the region.