Cancer of Unknown Primary: Introduction  

What is cancer?

Cancer is when cells in the body change and grow out of control. To help you understand what happens when you have cancer, let's look at how your body works normally. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them, and die when your body does not need them any longer.

Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in the body long enough, they can grow into (invade) nearby areas. They can even spread to other parts of the body (metastasis).

What is cancer of unknown primary?

The place in your body where cancer cells first start to grow is known as the site of the primary tumor. Cancer of unknown primary (CUP) is cancer that starts in an unknown area and has already spread by the time it’s diagnosed. Another name for CUP is unknown primary cancer or UPC.

The following are the places where CUP is often first found:

  • Lymph nodes in your neck, arms, or groin

  • Lungs

  • Liver

  • Brain

  • Bones

Healthcare providers normally name cancers for the part of the body in which they start. For instance, breast cancer is cancer that first starts growing in the breast. If breast cancer spreads to other parts of your body, it’s still called breast cancer. The cancer cells in other parts of your body look and respond to treatment like breast cancer cells. With CUP, however, doctors cannot tell which part of the body the cancer cells first came from.

CUP can affect many organs in the body. But the organ in which it first started may stay unknown. 

Sometimes, after many tests, healthcare providers will be able to find where the cancer started. Then they will rename the cancer for that site. They may also be able to figure out where the cancer began by tracking how it spreads.

Often in CUP, the only evidence that someone has cancer is a single site where the cancer cells have spread. This site in the body may be very far from the original location where the cancer cells first grew. To find the source of the cancer, healthcare providers take a sample of the cancer cells. This is called a biopsy. How the cells look under the microscope may help healthcare providers find the primary site. Knowing the primary site makes it easier to treat the cancer.

Many people with CUP have adenocarcinoma. These are cancers that start in the gland cells in the lining of organs. It may start in the glands in the linings of the lung, pancreas, stomach, colon, or other places. In other cases, CUP may be traced back to primary breast or prostate cancer.

In rare cases, your healthcare providers may not be able to figure out where your cancer started or where it’s spreading to. They may decide further testing would not be helpful. If this happens, they will treat your cancer based on the type of cancer that’s most likely in your case.

Talk with your healthcare provider

If you have questions about CUP, talk with your healthcare provider. He or she can help you understand more about your cancer.