Cancer can develop anywhere in the body. The organ or region of the body where cancer begins is known as the primary site. Cancer — including cancer that metastasizes, or spreads, to form new tumors elsewhere in the body — is named after the primary site. For example, colon cancer that spreads to the liver is called metastatic colon cancer, rather than liver cancer, because it contains colon cancer cells.

According to the American Cancer Society, an estimated 2 to 5 percent of all cancer patients have metastatic (secondary) tumors for which routine testing cannot locate the primary site. This is called cancer of unknown primary origin (CUP). Patients may be diagnosed with cancer of unknown primary origin if the primary tumor is too small to be identified with routine imaging tests, it regresses (disappears) before a secondary tumor arises, or the secondary tumor has several possible primary sites. Occasionally, a primary tumor is discovered during surgery to treat other conditions.

Cancer of unknown primary origin can appear anywhere in the body, but is most commonly found in the lymph nodes, liver, lungs, bones, or skin.