Cancer can develop anywhere in the body. The organ or part 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. This is because it contains colon cancer cells.

By definition, cancer of unknown primary (CUP) has spread from another part of the body. According to the American Cancer Society, an estimated 2 to 5 percent of all people with cancer have metastatic tumors for which routine testing cannot locate where the cancer started. Also called occult primary cancer, cancer of unknown primary is most commonly found in the lymph nodes, liver, lungs, bones, or skin.

A person may be diagnosed with cancer of unknown primary if the original tumor is too small to be identified with imaging tests or if the cancer is in many parts of the body and it is unclear which location is the primary site.

Every year, hundreds of people with a CUP diagnosis seek treatment at Memorial Sloan Kettering. Our team of experts uses sophisticated imaging technologies, advanced molecular profiling, and other laboratory tests to learn as much as possible about these tumors.

Sometimes, MSK’s doctors can determine the primary site through tissue review by expert pathologists, radiology imaging, or molecular testing, such as MSK-IMPACT™.

If the likely primary site is identified, treatment is directed there. For most people with CUP, a primary site of cancer is not identified. For them, our team of pathologists, medical oncologists, surgeons, and radiation oncologists identifies features of each individual tumor to determine the best care plan.