Cancer of unknown primary (CUP)
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).