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Cancer of unknown primary

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What is cancer of unknown primary (CUP)?

Cells in our body can sometimes change and no longer grow or behave normally. In some cases, changes to cells can cause cancer. Unlike normal cells, cancer cells can grow and divide out of control. These cancer cells can form lumps, or tumours. The area where the cells first change and cancer develops is called the primary site. Sometimes cancer cells can spread, or metastasize, from the primary site to other parts of the body. Cancer cells that spread are called metastasis.

Cancer of unknown primary (CUP) means that the cancer already spread before it was found, but doctors don’t know where it started. CUPcan be any one of many different types of cancer. About 50%–60% of all CUPs start in gland cells somewhere in the body. This type of cancer is called adenocarcinoma. CUP can also start in other types of cells in different parts of the body. These cancers can be lymphoma, melanoma, sarcoma, germ cell tumour, squamous cell carcinoma (SCC) or neuroendocrine carcinoma.

Doctors will do tests to try to find the primary site, but in many cases it is never found. This may be because the original tumour was very small and hard to find. It could also be because the body’s immune system destroyed the primary tumour after the cancer spread. When the primary site of CUP is found, it is most often the lung or pancreas. Other places where CUP may begin include the colon, rectum, kidney, liver, stomach, prostate, ovary, thyroid, breast, adrenal gland or parotid gland (a type of salivary gland).

CUP is uncommon. It makes up only 2%–5% of all cancers.


Brock Taraba Brock has been cancer free for over a decade, thanks to the support we received from the Canadian Cancer Society.

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