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What is carcinoma of unknown primary?

Carcinoma of unknown primary (CUP) is a disease in which cancer (malignant) cells are found somewhere in the body, but the place where they first started growing (the origin or primary site) cannot be found. This occurs in about 2%-4% of cancer patients.

Actually, CUP can be described as a group of different types of cancer all of which have become known by the place or places in the body where the cancer has spread (metastasized) from another part of the body. Because all of these diseases are not alike, chance of recovery (prognosis) and choice of treatment may be different for each patient.

If CUP is suspected, a doctor will order several tests, one of which may be a biopsy. This means a small piece of tissue is cut from the tumor and looked at under a microscope. The doctor may also do a complete history and physical examination, and order chest x-rays along with blood, urine, and stool tests. A cancer can be called CUP when the doctor cannot tell from the test results where the cancer began.

The pattern of how CUP has spread may also give the doctor information to help determine where it started. For example, lung metastases are more common when cancer begins above the diaphragm (the thin muscle under the lungs that helps the breathing process). Most large studies have shown that CUP often starts in the lungs or pancreas. Less often, it may start in the colon, rectum, breast, or prostate.

An important part of trying to find out where the cancer started is to see how the cancer cells look under a microscope (histology). Other special tests may also be done that help the doctor find out where the cancer started and choose the best type of treatment.

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