Skip to main content

Approved by the Cancer.Net Editorial Board, 10/2022

What is cancer?

Cancer is a group of more than 100 different diseases. Cancer begins when healthy cells change and grow out of control, forming a mass called a tumor. A tumor can be cancerous or benign. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. A benign tumor can grow but will not spread.

Doctors are usually able to figure out where a cancer began, known as the primary site. Then they identify any secondary or metastatic site if the cancer has spread. No matter where the cancer spreads, it is still named after the area of the body where it began. For example, colorectal cancer that has spread to the lungs is called metastatic colorectal cancer, not lung cancer.

What is cancer of unknown primary?

For about 2% of people diagnosed with cancer, the cancer is found at a secondary site, but routine testing cannot help doctors identify where the cancer began. These cancers are called “carcinoma of unknown primary site” or “cancer of unknown primary” (CUP).

For some people with CUP, specialized testing can eventually help identify the primary site. However, sometimes the primary site cannot be identified. This may be because:

  • The primary tumor is still very small.
  • The body caused the primary tumor to shrink or disappear.
  • The primary tumor was removed during a previous surgery for another condition, such as the removal of a mole on the skin or surgery to remove uterus, known as a hysterectomy.

However, even when the primary tumor cannot be located, the tissue of origin can often be predicted using specialized pathologic and molecular testing of the cancer tissue obtained at the time of a biopsy. More information about this process is explained later in this guide.