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 less than 1 percent of all cases seen in the United States, according to the National Cancer Institute. It’s estimated that CUP will be diagnosed in about 34,950 people in the U.S. in 2024.

The primary cancer may not be found for one of the following reasons:

• The primary cancer is very small and grows slowly.
• The body’s immune system killed the primary cancer.
• The primary cancer was removed during surgery for another condition and doctors didn’t know cancer had formed. For example, a cancer in the uterus may be removed during a hysterectomy to treat a serious infection.

Cancer cells usually look like the cells in the type of tissue in which the cancer began. For example, breast cancer cells may spread to the lung, but, because the cancer began in the breast, the cancer cells in the lung look like breast cancer cells.

Carcinoma of Unknown Primary Treatment (PDQ®)

Source: National Cancer Institute