Metastatic is a term defining cancers that have spread from a primary site to other areas of the body. Metastatic cancers begin as localized tumor cells, which then break free and travel in the bloodstream or lymph vessels to end up in other organs or lymph nodes.

Metastatic tumors can mutate from what was the genetic make-up in the primary tumor. Most metastatic cancers are named for the body part where they began and not for where they end up, although it is possible that the primary cancer site is unknown. When the primary site is known, this typically determines the most appropriate treatment course for the patient. Characterizing cancer as metastatic, in fact, has become a part of the clinical and pathologic staging system used in cancer prognoses and treatment. For example, metastatic breast cancer is the term used to describe cancer found in the lungs, bone, liver or other organs that originated in the breast. For example, tumor cells found in the lungs would not be categorized as lung cancer unless the lungs were the primary cancer site.

Doctors are still trying to fully understand the mechanisms behind metastasis, specifically the patterns of dissemination. In some cases, the backward analysis of metastatic patterns can even help identify unknown primary sites. While some types of cancers are known for significant metastatic activity (spreading to 4 or more distant sites), others are less likely to metastasize. Breast cancer, lung cancer, and melanoma are primary sites with a high frequency of metastases, while liver cancer has a low metastasis rate. Furthermore, there is a notable difference in metastatic spread to various secondary sites: liver, lymph nodes, lungs, and bone are affected frequently; whereas the pancreas, ovaries, thyroid, bone marrow, spleen, and skin are rarely affected. High frequency metastases typically haveto do with areas of high blood circulation (cancer cells traveling in the blood) or the lymphatic system.