What is cancer of unknown primary (CUP)?

Cancer of unknown primary (CUP) affects 3% (3 in 100) of people diagnosed with cancers in the UK. It can affect people of all ages but is most common in people aged 60 or over.

Cancer of unknown primary means that your doctor is not sure where the cancer started. They have found a secondary cancer but have not been able to find the primary site. People with CUP sometimes have more than one secondary cancer.

How CUP is confirmed

It usually takes several tests to confirm that the primary cancer cannot be found and for CUP to be diagnosed.

When a doctor finds a tumour, it is not always clear what type of cancer it is. You will have tests and investigations to find out whether it is:

  • a primary cancer
  • a secondary cancer – the primary cancer might be identified after the secondary cancer
  • a CUP – a secondary cancer but the primary cancer cannot be identified.

When you’ve had only a few tests, your doctors may know that the cancer is a secondary cancer, but they may not be sure where the primary is. At this stage, it is called a malignancy of unknown primary origin (MUO).

A doctor who specialises in treating CUP will look at all the results and may arrange more detailed tests, such as a biopsy. This needs to be 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 a CUP.

Even when your doctors can’t be sure of the primary cancer, they may be able to suggest the most likely 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 still give the doctors more information about the cancer and this will help them plan your treatment.

Knowing where the cancer started helps the doctors to know the best types of treatment to use for the secondary cancer. Treatment usually depends on where the primary cancer started. For example, a lung cancer that has spread to the liver will be treated using lung cancer treatments. A cancer that starts in the liver (primary liver cancer) is treated using treatments that are for liver cancer.

With CUP, as the primary cancer isn’t known, it can be difficult to know what the best treatment might be.

The most important part of your care is to manage any symptoms to help you feel better. Your GP, the hospital doctors or the palliative care team can advise and help with controlling symptoms.

Why the primary cancer can’t be found

A primary cancer can’t always be found because:

  • It may be too small to be seen on scans, or may be hidden beside a larger secondary cancer.
  • It might have disappeared, even though it has spread to other parts of the body and the secondary cancer is still growing. This can sometimes happen if the body’s immune system has successfully got rid of it.

Back to Understanding

What is cancer?

Cancer is a disease of our cells. Sometimes cells become abnormal and keep dividing to make more abnormal cells.

Types of CUP

Doctors may be able to find out the type of cell the cancer started in. This can help them decide on the best treatment.