Cancer of unknown primary (CUP) is when a secondary cancer is diagnosed, but doctors can’t tell where the primary cancer started. The secondary cancer is named after the primary cancer. For example, a cancer that starts in the lungs and spreads to the liver is still a lung cancer. The secondary cancer in the liver is made up of lung cancer cells and not liver cells.

Secondary cancers are also usually treated according to the primary cancer. Knowing where the cancer started helps the doctors to know what types of treatment to use for the secondary cancer. For example, a lung cancer that has spread to the liver will be treated using lung cancer treatments. It’s treated differently to a cancer that starts in the liver (primary liver cancer).

With CUP, the primary cancer isn’t known. This means that treatment choices are often more difficult to make. (

For some patients the origin of the cancer will be found through further tests, however for many patients the primary cancer will never be identified and will remain unknown. A cancer diagnosis in itself is devastating for patients, families and friends, but for the cause of the cancer to be unknown is entirely overwhelming.

Patients diagnosed with CUP need expert coordinated multidisciplinary support, effective communication is vital between teams, patients and their families regarding treatment plans and tests.

Metastases can develop close to the primary tumor. Even so close that the metastasis surrounds the tumor, as it were. It is then difficult for the doctor to tell the difference between the primary tumor and the metastasis. The doctor thinks he or she only sees the metastasis. He or she does not know that the primary tumor is also located here.

The primary tumor is too small to find by the available diagnostics, such ultrasound, CT, MRI and PET scan.
The primary tumor has been successfully fought by the patient’s immune system, or has left the body through a natural route (may involve primary tumors in the digestive tract).