Cancer of unknown primary (CUP) origin is the diagnosis when metastatic cancer is found but the place where the cancer began (the primary site) cannot be determined. About 2 to 4 percent of all cancer patients have a cancer whose primary site is never identified.
Treatment for CUP depends on many factors, including where the metastatic cancer is found, what the cancer cells look like under a microscope, and the patient's age and general health. Recent advances in diagnostic techniques have improved doctors' ability to eventually find the primary site, even when the original diagnosis is CUP.
This answer is based on the source infromation from the the National Cancer Institute
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