Unknown Primary

Cancer begins when normal cells change and grow uncontrollably, forming a mass called a tumor. A tumor can be benign (noncancerous) or malignant (cancerous, meaning it can spread to other parts of the body).

Cancer is a group of more than 100 different diseases. In most people with cancer, it is easy for doctors to find the primary site (where the cancer began), and any secondary or metastatic site (if the cancer has spread). No matter where the cancer spreads, it is still named by the primary site. For example, breast cancer that has spread to the brain is called metastatic breast cancer, not brain cancer.

For about 2% of people diagnosed with cancer, the cancer is found at a secondary site but routine testing cannot help doctors find where the cancer began, called the primary site. These cancers are called carcinoma of unknown primary site or cancer of unknown primary (CUP). For some people, specialized testing can eventually find the primary site; however, sometimes it cannot. This may be because the primary tumor is still very small, the body caused the primary tumor to shrink or disappear, and/or the primary tumor was removed during previous surgery for another condition, such as the removal of a mole on the skin or a hysterectomy (surgery to remove a woman's uterus).

Find out more about basic cancer terms used in this section.

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