Cancer of Unknown Primary


Cancer of an unknown primary is a term used to describe a cancer that has metastasized (spread) to another part of the body but for which the primary (original) site is not known when the cancer is found.

For example, breast cancer can metastasize to the lungs and cause a new tumor to grow in the lungs. Usually, when the new tumor is found, tests reveal that the tumor has breast cancer cells in it, and doctors can identify the tumor as metastatic breast cancer instead of lung cancer. However, in some cases, doctors cannot identify the primary site even when they use sophisticated tests.

Every year, several thousand people are diagnosed with metastatic cancer of an unknown origin in the United States. In fact, from 2 percent to 4 percent of all cancer patients are diagnosed with a cancer whose primary site is never found.

One of the reasons the primary cancer site may be difficult to diagnose is that most patients with cancer of an unknown primary are usually found to have cancer cells known as adenocarcinoma, which means that the cells originate in the lining of various organs, including the lungs, breast, liver, stomach, prostate, rectum and colon.

Newer diagnostic techniques are now available that have improved doctors’ ability to locate the primary site of a cancer of unknown primary. In patients for whom the primary cancer site is eventually found, the most common primary sites are the lungs and pancreas.

Because cancer of an unknown primary can originate from many different cancers, its treatment depends on the types of cancer cells found on examination with a microscope, where the cancer is located and the patient’s age and physical condition. Chemotherapy, radiation therapy, hormone therapy and surgery may all be involved, either alone or in combination.