What Is Carcinoma of Unknown Primary Origin?

The location in the body where cancerous cells first start to grow is known as the site of the primary tumor. Once cancer spreads to other parts of the body, it is said to have metastasized. Carcinoma of unknown primary origin (CUP) is cancer that starts in an unknown location and has already spread by the time it is diagnosed. Another name often used for CUP is unknown primary cancer, or UPC. These are the places where CUP is often first found:

  • Lymph nodes in your neck, arms, or groin

  • Lungs

  • Liver

  • Brain

  • Bones

Usually doctors name cancers for the part of the body in which they begin. For example, breast cancer is cancer that first starts growing in the breast. If breast cancer spreads to other parts of the body, it is still called breast cancer. The cancerous cells in other locations still resemble breast cancer cells. With CUP, however, doctors cannot tell which part of the body the cancer cells originally came from.


Carcinoma of unknown primary origin can affect many organs in the body, though the organ in which it originated may remain unknown. 

Sometimes, after several tests, doctors will be able to find where the cancer began and will rename the cancer for that site. Also, doctors may 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 cancerous cells have spread. This site in the body may be very far from the original location where the cancer cells first grew. To determine the source of the cancer, doctors take a sample of the cancer cells, called a biopsy. How the cells look under the microscope may help doctors find the primary site. This makes it easier for your doctor to suggest appropriate treatments.

Many people with CUP have a type of cancer cell known as adenocarcinoma. Adenocarcinomas 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 locations. In other cases, CUP may be traced back to primary breast or prostate cancer.

Your doctors may not be able to figure out where the cancer began or determine the path of spread it is taking. They may decide further testing would not be helpful. In these cases, they will proceed with treatment based on what type of cancer is most likely in your particular case.

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