Carcinoma of unknown primary is a diagnosis given when doctors can't determine where a cancer began. Cancers form from mutated cells in one area of the body and can spread (metastasize) to other areas. In carcinoma of unknown primary, the metastasized cells are discovered, but the tumor where they began can't be found.
Carcinoma of unknown primary is a rare and often aggressive type of cancer. Knowing where a cancer began plays a role in determining the best treatment. For this reason, doctors who treat people with carcinoma of unknown primary work to locate the organ where the cancer began. When that can't be determined, doctors use as many clues as they can gather to select the most appropriate treatment for carcinoma of unknown primary.