Staging is a way of describing or classifying a cancer based on the extent of cancer in the body. Different staging or classification systems are used for different cancer sites and types. With cancer of unknown primary (CUP), the person has metastatic cancer when they are diagnosed, so the cancer is at an advanced stage.
People with CUP are often considered to have a stage IV cancer because the cancer has spread beyond the original or primary site. However, this can depend on the type of primary cancer.
It is difficult to accurately stage CUP because often the primary site is not known.