Unknown primary cancer

If you or someone you know has been diagnosed with an unknown primary cancer, we can provide the information you need, from understanding the cancer itself, to choosing the right treatment, to finding support.

What you should know about unknown primary cancer

  • A primary cancer is where the cancer first grows in your body. A secondary cancer is when a primary cancer spreads to another area of your body.
  • If you are diagnosed with secondary cancer but your doctor is unsure where it began in your body, the first cancer is called an unknown primary cancer.
  • The symptoms of an unknown primary cancer depend on where the cancer is found in your body.
  • An unknown primary cancer can be hard to diagnose. Tests include X-rays, scans, blood tests and tissue samples (biopsies).
  • The main treatments for an unknown primary cancer are surgery, chemotherapy, radiotherapy and biological therapy.

What is the difference between primary and secondary cancers?

To explain what an unknown primary cancer is, you need to understand the difference between a primary and secondary cancer. A primary cancer is where the cancer first grows in your body. A secondary cancer is when a primary cancer spreads to another area of your body. A secondary cancer is also known as metastases. For example, if you are diagnosed with primary breast cancer and secondary bone cancer, it means that your cancer started in your breast and moved to your bones. If we took a sample of the cancer cells that had spread to your bone, they would be breast cancer cells rather than bone cancer cells. You would be suitable for breast cancer treatment and not treatment for bone cancer. If you had primary bone cancer, you would need a different treatment.

What is an unknown primary cancer?

It is rare to be diagnosed with an unknown primary cancer. Symptoms usually help your doctor to diagnose where the cancer started. Often it is secondary cancer that is found first, not the primary cancer. Secondary cancer cells are often hard to diagnose. This can leave your doctors unsure as to where the primary cancer is.

Why is a primary cancer not found?

There are some reasons why a primary cancer cannot be found. The cancer may have spread so quickly that the secondary cancer has grown larger than the primary cancer. The primary cancer might not be seen on a scan. Some doctors believe that your body’s immune system destroys the primary cancer but cannot destroy the secondary cancer. Or the primary cancer may be tiny and hidden by nearby secondary cancers on a scan.

How common is unknown primary cancer?

Unknown primary cancer is very rare.

Cancer publications

For booklets and factsheets, including information about cancer types, treatments, side-effects, emotional effects, financial information and more. Visit our publications section.

Useful resources

Note: Links to external websites are listed below. The Irish Cancer Society is not responsible for the contents of external websites.

Call our National Cancer Helpline

Freephone 1800 200 700 to talk to a specialist cancer nurse
It's open Monday-Thursday from 9am to 7pm and Friday from 9am to 5pm