Carcinoma of unknown primary is a diagnosis given when doctors can't determine where a cancer began. Carcinoma of unknown primary is rare, and it can be an aggressive type of cancer.
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.
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. As researchers are improving molecular testing and other innovative laboratory testing done on cancer cells, the need to locate the primary site may become less important.