Carcinoma of unknown primary is a diagnosis given when doctors aren't able to locate where a cancer began.

Most often, cancer is diagnosed when doctors discover the spot where the cancer began (primary tumor). If the cancer has spread (metastasized), those sites might be discovered, too.

In carcinoma of unknown primary, also known as occult primary cancer, doctors find the cancer cells that spread in the body, but they can't find the primary tumor.

Doctors consider the location of the primary tumor when choosing the most appropriate treatments. So if carcinoma of unknown primary is found, doctors work to try to identify the primary tumor site. Your doctor might consider your risk factors, symptoms, and results from exams, imaging tests and pathology tests when trying to determine where your cancer began.


Signs and symptoms of carcinoma of unknown primary depend on what part of the body is involved. In general, they might include:

  • A lump that can be felt through the skin
  • Pain
  • Changes in bowel habits, such as new and persistent constipation or diarrhea
  • Frequent urination
  • Cough
  • Fever
  • Night sweats
  • Losing weight without trying


In general, cancer forms when cells develop changes (mutations) in their DNA. The DNA contains instructions that tell cells what to do. Certain mutations can cause a cell to multiply uncontrollably and to continue living when normal cells would die. When this happens, the abnormal cells accumulate and form a tumor. The tumor cells can break away and spread (metastasize) to other parts of the body.

In carcinoma of unknown primary, the cancer cells that spread to other parts of the body are found. But the original tumor isn't found.

This can happen if:

  • The original cancer is too small to be detected by imaging tests
  • The original cancer was killed by the body's immune system
  • The original cancer was removed in an operation for another condition

Risk factors

The risk of carcinoma of unknown primary might be related to:

  • Older age. This type of cancer is most likely to occur in people older than 60.
  • Family history of cancer. There's some evidence that carcinoma of unknown primary might be associated with a family history of cancer that affects the lungs, kidneys or colon.

Carcinoma of unknown primary care at Mayo Clinic

Oct. 06, 2022

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  1. Occult primary (cancer of unknown primary [CUP]). Plymouth Meeting, Pa.: National Comprehensive Cancer Network. https://www.nccn.org/professionals/physician_gls/f_guidelines.asp. Accessed Nov. 7, 2018.
  2. Niederhuber JE, et al., eds. Carcinoma of unknown primary. In: Abeloff's Clinical Oncology. 5th ed. Philadelphia, Pa.: Churchill Livingstone Elsevier; 2014. https://www.clinicalkey.com. Accessed Nov. 7, 2018.
  3. Carcinoma of unknown primary treatment (PDQ)–Patient version. National Cancer Institute. https://www.cancer.gov/types/unknown-primary/patient/unknown-primary-treatment-pdq. Accessed Nov. 7, 2018.


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