Cancer of unknown primary origin

Cancer of unknown primary origin (CUP, "occult cancer") is a term used to describe a cancer that is said to be at the metastatic stage at the time of diagnosis, but a primary tumor cannot be identified. A diagnosis of CUP requires a clinical picture with metastatic disease and one or more biopsy results inconsistent with a primary tumor.
CUP is found in about 3 to 5% of all people diagnosed with invasive cancer, and carries a poor prognosis in most (80 to 85%) of those circumstances. The other 15 to 20% of patients, however, have a relatively long survival with appropriate treatment.