Carcinoma of Unknown Primary

Sometimes doctors find where a cancer has spread but cannot find where in the body the cancer first began to grow. This type of cancer is called a cancer of unknown primary (CUP) or occult primary tumor.

CUP is a rare category of cancer, accounting for around 2% of all cases seen in the United States. It’s estimated that CUP will be diagnosed in about 34,950 people in the U.S. in 2024.

Some of the reasons the primary tumor may not be found are it is very small and grows slowly, the body’s immune system already killed it, or it was removed during surgery for another condition and the doctors were unaware cancer had formed.  

Only about 30% of patients who are diagnosed with CUP are alive after one year, according to the National Cancer Institute. Chance of recovery can be impacted by a number of factors including where the cancer began and where it has spread, the number of organs with cancer, and whether the cancer has recurred or if it is an initial diagnosis.   

Carcinoma of Unknown Primary Treatment (PDQ®)

Source: National Cancer Institute