Definition & Overview

Metastatic cancer of unknown primary, also referred to as cancer of unknown primary (CUP), or occult primary cancer, is a cancer that is found at a metastatic site but the primary site is unknown. Cancer usually begins at a primary site, such as the lungs and breast. As the cancer spreads throughout the body, it can affect other organs. The affected organs are referred to as the metastatic sites.

Metastatic cancers follow the name of the primary cancer. For instance, if the primary site is the breast, the cancer is referred to as breast cancer. If it metastasizes in the lungs, it is still classified as breast cancer.

However, in some cases, doctors are unable to determine the primary site of the cancer, so they refer to it as a cancer of unknown primary. This type of cancer may not be as common as other types, but they still account for around 3-5 percent of the total cancer cases. They are also included in the top 10 most common malignancies in developed societies.

In a majority of CUP cases, the primary site was never identified. In fact, even autopsies never revealed where the cancer began. This is unfortunate because doctors rely heavily on identifying the primary cancer to provide the right form of treatment.

Without the knowledge of the primary type of cancer, the prognosis of CUP is often poor. According to the National Cancer Institute, the median survival period is 3 to 4 months. Less than 25% of CUP patients are able to survive for a year. Less than 10% are able to survive for the next 5 years.

Cause of Condition

The generally accepted origin of cancer is the growth of an abnormal cell. As the cells continue to grow instead of dying off at a certain stage, they form a tumor. A malignant tumor will then spread to other areas. In some cases, instead of growing further, the primary tumor remains small. As such, current imaging technologies are not able to detect it.

It is almost certain that CUP does have a primary site. In some CUP cases, further diagnostic procedures were able to detect the primary tumor. As imaging and detection technologies continue to improve, the number of CUP cases is also expected to decrease.

In cancer types where the primary site was identified, it’s possible to determine the exact cause or risk factors for that type of cancer. In CUP types, it would be highly unlikely that the exact cause will be known. Therefore, it would also be impossible to identify specific risk factors.

However, if doctors can identify a pattern of metastases, the chances of locating the primary site somewhat improve because cancers are known to metastasize in different ways. For example, lung metastases are known to have primary sites located above the diaphragm. If the metastasis is found in the liver, then it is likely that the primary site will be below the diaphragm. In some cases, identifying the metastasis pattern will help in deciding upon a treatment method that would likely result in a better response.

Key Symptoms

The symptoms of CUP largely depend on the location of the metastasis. For instance, in a lung metastasis, the patient will display symptoms such as shortness of breath or breathing difficulties. If the metastasis is located in the liver, the likely symptoms would be jaundice and abdominal swelling. Metastasis in the bone will result in pains and could even lead to bone fracture. However, not every patient with metastatic cancer will display symptoms. In fact, for many patients, the metastases were only located during an x-ray or other imaging tests.

Who to See & Types of Treatments Available

It is unfortunate that the majority of CUP cases were only identified in its late stages. Rarely would a CUP case be identified while it is still in its early stages. The main reason is that unlike other types of cancers, CUP does not have early detection procedures.

Patients with CUP would only be diagnosed as such if they requested a consultation for cancer-related symptoms, such as breathing difficulties and abdominal swelling. It could also be possible that the metastatic cancer was identified during a routine medical checkup or treatment for an unrelated complaint.

If your doctor suspects that you may have cancer, you’ll likely be referred to a cancer specialist called an oncologist. The oncologist will attempt to correctly identify the cancer. If he or she determines that a particular tumor is metastatic, then there will be attempts to locate the primary tumor. If these attempts are unsuccessful, you’ll be diagnosed with CUP.

It’s important to understand that the majority of CUP cases are diagnosed when they are already in the advanced stages. It is possible that the cancer will even be too advanced to be cured. However, treatment will still be provided, but instead of trying to cure the disease, the ultimate goal would be to shrink it or prevent it from spreading. Doing so would help in reducing the symptoms and may even extend life expectancy.

Treatment for CUP may include surgery, medications, radiation therapy, targeted therapy, chemotherapy, and hormone therapy. It would be advisable to obtain as much information about your condition and the recommended treatments from your doctor.


  • Perry MC. Approach to the patient with cancer. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 182.
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