Cancer of Unknown Primary

Cancer of unknown primary origin (CUP) is the diagnosis when metastatic cancer is found but the place where the cancer began (the primary site) cannot be determined. Cancer can begin in any organ or tissue of the body. The primary, or original, tumor is usually named for the part of the body or the type of tissue in which the cancer begins. The disease may spread (metastasize) from the primary tumor and form metastatic tumors in other parts of the body. For example, breast cancer cells can metastasize to the lungs and cause the growth of a new tumor. When this happens, the tumor in the lung is called metastatic breast cancer because it is composed of breast cancer cells, not lung cancer cells.

About two to four 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' abilities to find the primary sites, even when the original diagnoses are "CUP."

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