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Cancer of Unknown Primary Basics

About Cancer of Unknown Primary

Cancer of unknown primary refers to cancer that has spread in the body and its source of origin cannot be found.  

  • The source of origin, where the cancer originally started, is also called the primary site.
  • Cancer that has spread is called metastatic cancer. All cancers spread at different rates.

Reasons why the primary site for a metastatic cancer cannot be found vary. 

In some situations, while the type of cancer cell can be determined, that type of cell is not exclusive to the primary site. For example, the most common cancer of unknown primary is called “poorly differentiated adenocarcinoma of unknown primary.” Adenocarcinoma is a type of cancer cell that can occur in the breast, lung, colon, pancreas, uterus and other organs.

In other situations, the cancer cells are very primitive looking (immature) and do not look anything like the original tissue where they developed.  

In addition, according to the National Cancer Institute, reasons can include:

  • 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 uterus with cancer may be removed during a hysterectomy to treat a serious infection.


How Treatment is Determined 

Metastatic cancer cells are the same as the cells in the primary tumor. For example, breast cancer that spreads to the lungs is still breast cancer, and it is called metastatic breast cancer. This is important to understand because cancer treatment is determined by the primary cancer. Metastatic breast cancer located in the lung receives treatment that is known to fight breast cancer cells.

When doctors find cancer that has spread, but they cannot determine where it started, treatment plans are developed based on the type of cells in the metastatic cancer, its location, size of the tumors and many other factors individual to the cancer patient. Surgery, chemotherapy and radiation are common treatment options for cancer of unknown primary.

To learn more about cancer of unknown primary, we recommend visiting these websites:

National Cancer Institute (

American Cancer Society (