Unknown primary cancer

Cancer of unknown primary (CUP) is when a secondary cancer is diagnosed, but even after tests have been carried out, doctors can't tell where the cancer first started. The primary cancer is unknown.

Cancer of unknown primary (CUP) is a term that covers many different types of cancer. It affects about 3-5% of people with cancer.

People with CUP often have more than one secondary cancer. CUP is more common in people aged 60 or over, but it can affect people younger than this.

When a new tumour is found, it's not always obvious what type of cancer it is. You'll have tests and investigations to find out whether it is:

  • a primary cancer
  • a secondary cancer - the primary cancer is then identified after the secondary cancer
  • a true CUP - when it's not possible to identify a primary.

When you've had only a few tests, your doctors may know that the cancer is a secondary, but they may not be sure where the primary is. This is called a malignancy of undefined primary origin (MUO).

If they still can't find a primary after more tests, the cancer may then be described as a provisional CUP(pCUP).

You will then have more detailed tests and a doctor who specialises in the treatment of CUP will look at all the results. This is done before a diagnosis of confirmed CUP(cCUP) can be made.

Sometimes, tests will find the primary cancer. When this happens, the cancer is no longer called CUP.

If your doctors can't be sure of the primary cancer, they may be able to suggest a possible part of the body where the cancer started. This will be based on where the secondary cancers are, your symptoms and the test results. The test results will also suggest how the cancer might behave. This will help your specialist to plan your treatment.

Why the primary cancer can't be found

There are different reasons why a primary cancer can't always be found:

  • It may be too small to be picked up on scans or be hidden beside a larger secondary cancer.
  • It might have disappeared, even though it has spread to other parts of the body. This can sometimes happen if the body's immune system has successfully got rid of it.
  • It might have been passed out of the body. For example, a small cancer in the wall of the bowel may become detached and leave the body in the bowel motions (stools).

Symptoms of unknown primary cancer

Symptoms in different areas of the body

  • Secondary cancer in the lung - You may have a cough that doesn't go away, breathlessness or fluid collecting around the lungs (pleural effusion).
  • Secondary cancer in the bone - You may experience a dull, persistent pain in the bone that's often worse at night. Sometimes if a bone is weakened by cancer it will break (fracture).
  • Secondary cancer in the liver - Symptoms include swelling and discomfort in the tummy (abdomen), feeling sick and loss of appetite, yellowing of the skin and eyes (jaundice), and fluid collecting in the abdomen (ascites).
  • Secondary cancer in the lymph nodes (glands) - Your lymph nodes may be swollen and feel hard or cause pain if they're pressing on tissue or nerves nearby. The most common lymph nodes to be affected are the nodes in the neck, armpit, chest or groin.

General symptoms

You may also have some general symptoms including:

  • unexplained weight loss
  • loss of appetite
  • feeling extremely tired all the time
  • looking pale, feeling tired and breathless due to a lack of red blood cells (anaemia).

All the symptoms mentioned here can be caused by conditions other than cancer, but it's important to see your doctor and get them checked.

*Information provided by Macmillan cancer support 

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