There is no staging system for cancer of unknown primary (CUP). This is because the cancer has already spread.
What staging is
When someone is diagnosed with cancer, doctors usually carry out a number of tests to find out the size of the tumour and whether it has spread to other areas of the body. This is called staging. It helps doctors to decide on the best treatment plan.
Different cancer types have slightly different staging systems. So the stages used for bowel cancer are different from the stages used for breast cancer, for example.
For most types of cancer there are 4 number stages, from 1 to 4. Stage 1 is a small, localised tumour (early cancer). Cancer that has spread to another part of the body is a stage 4 cancer (advanced cancer).
Cancer of unknown primary
With cancer of unknown primary, the cancer has already spread and the doctors don’t know where the original cancer started. So they can't accurately stage this type of cancer.
You usually still have several tests to find out where the cancer is, if it’s just in the lymph nodes, or if it’s in one or more body organs. Your doctor uses all the information they can gather to help decide on your best treatment options.
The results of your scans and tests can often suggest the most likely place the cancer started.