Metastatic breast cancer is breast cancer that has spread to another part of the body. Cancer cells can break away from the original tumor in the breast and travel to other parts of the body through the bloodstream or the lymphatic system, which is a large network of nodes and vessels that works to remove bacteria, viruses, and cellular waste products. (See the Breastcancer.org page on lymph nodes for a complete explanation of the lymphatic system.)
A metastatic tumor in a different part of the body is made up of cells from the breast cancer. So if breast cancer spreads to the bone, the metastatic tumor in the bone is made up of breast cancer cells, not bone cells.
Breast cancer can be "metastatic at first diagnosis," or "de novo metastatic." This means that the cancer in the breast wasn't detected before it spread to another part of the body.
Metastatic breast cancer can be recurrent breast cancer if the original cancer has come back and spread to another part of the body.
Metastatic breast cancer is considered advanced-stage breast cancer.
Breast cancer can come back in another part of the body months or years after the original diagnosis and treatment. Nearly 30% of women diagnosed with early-stage breast cancer will develop metastatic disease. In developing countries, most women diagnosed with breast cancer are diagnosed with advanced-stage or metastatic disease.