Overview

Cancer is a group of more than 100 different diseases characterized by the uncontrolled, abnormal growth of cells. Normally, the body can kill abnormal cells or stop them from dividing, but sometimes they keep growing and form a tumor. Cancer can move through the blood and lymph systems and spread to other parts of the body.

In most people with cancer, it is easy for doctors to identify the primary site (where the cancer began), and the secondary or metastatic site (if the cancer has spread and where). No matter where the cancer spreads to, the cancer is still described by the primary site. For example, breast cancer that has spread to the brain is called metastatic breast cancer, not brain cancer.

In less than 5% of all people diagnosed with cancer, the cancer is found at a metastatic (secondary) site but routine testing cannot locate the site where the cancer began. These cancers are called carcinoma of unknown primary site or cancer of unknown primary (CUP). In some people, specialized testing can identify the primary site; however, in many cases, it cannot. This may be because the primary tumor remains very small, the body is able to cause the primary tumor to shrink or disappear, and/or the primary tumor was removed during previous surgery for another condition, such as a mole on the skin or a hysterectomy (surgery to remove a woman’s uterus).

Statistics

CUP accounts for approximately 2% to 5% of cancers diagnosed in the United States each year. The average age of a person diagnosed with this type of cancer is about 60.

People with CUP are a diverse group, and prognosis (chance of recovery) varies widely. Often, treatments are limited for people whose cancer is widespread at the time of diagnosis. However, others have a cancer that can be successfully treated; for some, curative therapy is available. These differences and survival rates are discussed in the Treatment section.

Cancer survival statistics should be interpreted with caution. Estimates are based on data from thousands of cases of this type of cancer in the United States each year, but the actual risk for a particular individual may differ. It is not possible to tell a person how long he or she will live with CUP.

Statistics adapted from the American Cancer Society.

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