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. CUP is a term that covers many different types of cancer. People with CUP sometimes have secondary cancer in more than 1 part of the body.
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 - where the cancer started
- a secondary cancer - the primary cancer is then identified after the secondary cancer
- a 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 unknown 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.
With CUP, doctors cannot always tell which part of the body the cancer cells are from. But cancers are made up of different types of cells. These can be grouped based on the type of cells they are made up of. Knowing the type of cell can give the doctors a better idea about where the cancer may have started. This helps them to plan the best treatment.
Why the primary cancer can't be found
There are different reasons why a primary cancer can't always be found:
- the primary cancer is too small to be seen on scans
- the primary cancer is hidden by a larger secondary cancer
- the body’s immune system has destroyed it, but it has already spread.
Doctors might stop looking for the primary cancer if:
- you are not feeling well enough to have tests
- further test results are unlikely to change your treatment plan.
Symptoms of unknown primary cancer
Symptoms of cancer of unknown primary (CUP) depend on where the secondary cancer (or cancers) is in the body. You may also have some general symptoms including:
- unexplained weight loss
- loss of appetite
- feeling extremely tired all the time
- pain that does not go away or has no obvious cause
- looking pale, and feeling tired and breathless – this may be caused by a low level 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
Depending on the type of tumour the CUP is, the main treatment might be surgery to remove it. Chemotherapy or radiotherapy might also be suitable treatment options.