What is cancer of unknown primary?

Cells in our body can sometimes change so they no longer grow or behave normally. In some cases, changes to cells can cause cancer. Unlike normal cells, cancer cells can grow and divide out of control. These cancer cells can form lumps, or tumours.

The area where the cells first changed and cancer developed is called the primary site. Sometimes cancer cells can spread, or metastasize, from the primary site to other parts of the body. When cancer cells spread, it is called metastasis.

Cancer of unknown primary (CUP) means that cancer is found in the body and it is difficult to tell where it started. It may also be called occult primary or cancer of unknown primary origin.

Doctors will do tests to try to find the primary site, and it is often found. In most cases, when doctors find the primary site of CUP it is the lung or pancreas. CUP can also start in other organs.

In a small number of cases the primary site can’t be found, so the cancer is called CUP. Doctors may not be able to find the primary site because the original tumour is very small and doesn’t show up on any diagnostic tests. Sometimes cancer cells don’t form a tumour before they spread to another part of the body.

CUP makes up 2% to 5% of all cancers. In these cases the primary site cannot be identified even when all appropriate tests have been done.