What is cancer of unknown primary?

Our bodies are made up of millions of cells. When cells behave normally, we stay healthy. Unlike normal cells, cancer cells can grow and divide out of control. These cells can form lumps, or tumours, which may eventually spread to other tissues and other parts of the body. Cancer cells that spread are called metastasis.

Cancer of unknown primary (CUP) means that the cancer has already spread (metastasized) when it’s found, but doctors don’t know where it started (primary site). Sometimes the primary site is identified after further testing, but in many cases the primary site is never found.

CUP is not a single type of cancer, but a group of different types of cancer. Cancer of unknown primary may also be called carcinoma of unknown primary site or occult primary cancer.

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We can give information about cancer care and support services in Canada only. To find a cancer organization in your country, visit Union for International Cancer Control or International Cancer Information Service Group.