Trying to find where cancer of unknown primary started

You have tests to look for the cause of your symptoms. These may show that you have cancer spread (secondary cancer), but your doctor can’t find where it started (primary cancer). When this happens, you are usually referred to a team of professionals who specialise in cancer of unknown primary (CUP).

The CUP team might arrange a number of further tests to try to work out where the cancer started. You are diagnosed with a confirmed cancer of unknown primary (cCUP) if doctors still can’t find the primary cancer.

Sometimes doctors find the primary cancer at a later date. When this happens, the cancer is no longer a CUP and your treatment will follow the guidelines for the specific cancer type.

How doctors decide what tests to do

Whereabouts the secondary cancer is in your body can help your doctors decide what tests to do. For example, secondary cancer in lymph nodes in the neck most likely came from a primary cancer in the head and neck area. So doctors might arrange for you to have a scan such as PET-CT scan. 

Tests on the tissue sample

Your team might do further tests on the sample of tissue (biopsy) used to diagnose your cancer. They might use immunohistochemistry (IHC) tests, which look for particular proteins on the surface of the cancer cells. These tests can sometimes tell which type of cell the cancer started in.

Looking at genes or molecules in the cancer tissue

Techniques called gene expression profiling and molecular profiling are now available for research purposes. But they are not routinely available on the NHS. Researchers need to find out more about how well these tests work to diagnose CUP.

Gene expression profiling aims to identify genetic patterns in cancer tissue. It looks at the patterns of genes in the secondary tumour to try to find out where the primary cancer is.

Molecular profiling looks at genetic material or particular molecules in the biopsy sample. Its techniques can be used to find out the type of cancer cell. These tests are sometimes available in private hospitals.

Some clinical trials are looking at molecular profiling for people diagnosed with CUP. Your doctor might suggest you enter a trial if one is available.

When doctors stop looking for the primary cancer

It may not be helpful to spend time doing further tests to try to find where the cancer started if you have a very advanced cancer. Your team might suggest starting treatment straight away to help control your symptoms.

Doctors might stop looking for the primary cancer when: 

  • you are not well enough to have treatment
  • the results of further tests are unlikely to change your treatment plan

Getting your results

Sometimes even after many tests, the primary cancer can’t be found. It is then called a confirmed cancer of unknown primary (cCUP). All the information your doctors have gathered about the cancer helps them decide on the best treatment for you.

Doctors continue to look for signs and symptoms that could tell them what type of primary cancer you have. These could show up at any time, even after treatment. If doctors find the primary cancer at a later date, the cancer is no longer a CUP. Your treatment will follow the guidelines for the specific cancer type. For example, tests showing that your cancer started in the stomach means that you have stomach cancer. It is then treated as such, even if the primary tumour can’t actually be seen in the stomach.

It can be very shocking to be told that you have cancer that has spread. Not knowing where the cancer started can make the news even harder to take in. Your treatment team, including your specialist nurse, will do all they can to support you.

You can speak to Cancer Research UK’s information nurses if you have questions about CUP. Contact them on freephone 0808 800 4040. The lines are open from 9am to 5pm, Monday to Friday.
Last reviewed: 
13 May 2021
Next review due: 
13 May 2024
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