Decorative image

About cancer of unknown primary

Cancer of unknown primary (CUP) means that cancer spread (secondary cancer) has been found in your body, but doctors don't know where the cancer started (the primary tumour).

What cancer is

Your body is made up of billions of cells that can only be seen under a microscope. The cells are grouped together to make up the tissues and organs of our bodies.

Normally, cells only divide to replace old and worn out cells. Cancer develops when something inside a single cell goes wrong, making the cell carry on dividing until it forms a lump or a tumour.

A tumour can be either non cancerous (benign) or cancerous (malignant). A benign tumour does not spread to other parts of the body. But a malignant tumour (cancer) can spread.

What a primary cancer is

The place where a cancer starts growing 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.

Cancers are named and treated according to where they first started developing, even if they have spread to other parts of the body.

For example, if you have bowel cancer that 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 actually cancerous bowel cells. They are not liver cells that have become cancerous. Doctors confirm this by looking at the cells under the microscope.

Diagram showing a primary and secondary cancer

Why a primary cancer might not be found

It's normally straightforward to find a primary cancer. Usually, people go to their GP because of symptoms and are referred to a specialist doctor for tests which confirm the cancer. Sometimes secondary cancers are found in one or more parts of the body, but despite a number of tests the doctors can't find the primary site. This is called a cancer of unknown primary (CUP). It can also be called an occult cancer.

There are different reasons why a primary cancer can't be found, including:

  • 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 lymph nodes, bone, brain, liver and skin.

How doctors know you have cancer of unknown primary

Doctors can often tell the type of cancer by what the cells look like under a microscope. For instance, cells taken from a tumour in the lung might look like breast cancer cells. So the doctor knows it is a 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. Cells like this are known as poorly differentiated or undifferentiated.

This can make it very difficult for the doctor to tell what kind of cell the cancer started from. In such a case, the cancer is called a poorly differentiated cancer of unknown primary.

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

How common it is

Around 8,700 people are diagnosed with cancer of unknown primary in the UK each year. That's about 2 out of every 100 cancers diagnosed (2%). It's a more common situation than many people realise.

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.

Information and help