Cancer of unknown primary (CUP)

If you are diagnosed with cancer of unknown primary (CUP), it means that your doctor cannot find where your cancer started.

Cancer occurs when normal cells grow and change in an abnormal and uncontrolled way. When groups of these cancer cells continue to grow, they can form a lump called a (primary) tumour. Tumours can spread to other parts of your body through your blood or lymphatic system.

If the primary tumour spreads to other parts of the body and forms a new tumour, this is known as secondary cancer or metastasis. CUP means that your doctors have found a secondary cancer but they cannot find the primary cancer. In other words, they have found where your cancer has spread to, but they cannot find the place in your body where the cancer began.

There are a number of reasons why a primary cancer may not be found. These include:

• It may be too small to be seen on scans.
• It may be hidden by a larger secondary cancer.
• The body’s immune system may have already got rid of the primary cancer, but not the secondary cancer.

During your diagnosis, a small amount of tissue would have been taken from your body and examined under a microscope to find out if cancer cells were present. This is a biopsy. 

During a biopsy, a specialist known as a pathologist can tell if the sampled tissue does not belong to the surrounding tissue. This can then be confirmed by further tests. 

If you have cancer of unknown primary (CUP), this means that your cancer has already spread, but doctors don't know where it started. This makes it very hard for doctors to know what the specific risks for CUP are. 

As a result, people should consider general risks for cancer. These include:

Medical content updated from our 'Understanding cancer of unknown primary (CUP)' booklet (2024), reviewed by Dr Dearbhaile Collins, Consultant Medical Oncologist and Una Murphy, Daffodil Centre Nurse.