What normally happens when cancer develops
The place a cancer starts to develop is called the primary site. Cells from this primary site may break away and spread to other parts of the body through the bloodstream or lymphatic system. These escaped cells can then grow and form other tumours, which are known as secondary cancers or metastases. 
Doctors name and treat cancers according to where they first started developing, even if they have spread to other parts of the body.
For example, if a bowel cancer has spread to the liver, it is called bowel cancer with liver metastases or secondaries, it is not called liver cancer. This is because the cancerous cells in the liver are cancerous bowel cells, they are not liver cells that have become cancerous. Doctors confirm this by looking at the cells under the microscope.

Cancer of unknown primary (CUP) means that cancer spread has been found in your body (secondary cancer), but your doctors can't find where the cancer started (the primary cancer). It is sometimes called unknown primary cancer or unknown primary tumour.
The tests and treatments for cancer of unknown primary in this section is relevant to adults.


Why a primary cancer might not be found
Sometimes there are different reasons why doctors can't find a primary cancer. They don’t always know for certain why and how this happens. This might be because:

  • the secondary cancer has grown very quickly, while the primary cancer is still very small – very small cancers might not cause symptoms or be seen on scans
  • your immune system has successfully attacked the original primary cancer and it has disappeared, while the secondary cancer is still growing (this is not common, but it can happen)

The most common places for secondary cancers to be found are the lungs, liver, bones, lymph nodes, and skin.

How doctors know you have CUP
Doctors can often tell the type of cancer by what the cells look like under a microscope. For example, cells taken from a tumour in the lung might look like breast cancer cells. So the doctor knows it is breast cancer that has spread to the lung, rather than a cancer that started in the lung. 
Sometimes cancer cells don’t look like any particular type of normal cell. The cells are very abnormal and have not become specialised enough to look like breast cells or lung cells, for example. This can make it very difficult for the doctor to tell what kind of cell the cancer started from.


Types of cells and cancer
Most cancers are cancers of the epithelial cells. Epithelial cells are found in the skin or tissues that line or cover the internal organs. Cancers that start in epithelial tissue are called carcinomas. Most cancers of unknown primary are types of carcinoma.
Other types of cancer develop from different types of body cell. They include:

  • sarcomas, which develop from cells of the connective and supportive tissue, such as bones, muscle, fat, blood vessels or other soft tissues
  • leukaemias, which are cancers of white blood cells found in the bone marrow
  • lymphomas, which are cancers that begin in cells of the immune system
  • Click here to read more about different cell types and cancer

How common is CUP
Around 8,600 people are diagnosed with cancer of unknown primary in the UK each year. That's about 2 out of every 100 cancers diagnosed (2%). 


Who gets it
Cancer of unknown primary can develop at any age. But almost 60 out of 100 cases (almost 60%) are in people over the age of 75.

  • Click here to read more information about risks and caused of cancer of unknown primary. 
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