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What is cancer of unknown primary (CUP)?

Men and woman discussing unknown primary cancer

This page tells you what cancer of unknown primary (CUP) is. Cancer of unknown primary is also known as UPC or UPT.

 

A quick guide to what's on this page

What is cancer of unknown primary (CUP)?

Cancer of unknown primary means that you have a cancer, but your doctor is not sure where it started from. The place where a cancer starts growing in the body is called the primary site. Cells from this primary site can break away and spread to other parts of the body. They can then form other cancers, which are known as secondary cancers or metastases. 

Sometimes secondary cancers are found but the doctor can't find the primary site. This is called cancer of unknown primary (CUP or UPC). The secondary cancer may have grown quickly, whilst the primary cancer is still very small. Or the primary cancer may have disappeared while the secondary cancers are still growing.

Examining the cells

A specialist will examine your cancer cells under a microscope to try to find out what type of cell they are. But sometimes cancer cells don’t look like any particular type of normal cell. So the doctor can't tell which part of the body they came from.

 

CR PDF Icon You can view and print the quick guides for all the pages in the About CUP section.

 

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 tumour. You can find information about cells, how they multiply and what happens when it goes wrong, in the cells and cancer section. A tumour can be either benign or cancerous (malignant). A benign tumour does not spread to other parts of the body. But a malignant tumour (cancer) can spread to other parts of the body.

 

What a primary cancer is

The place where the cancer starts growing is called the primary site. If the cancer is not treated, cells from this primary site can break away and spread to other parts of the body. These escaped cells can then form other cancers, which are known as secondary cancers or metastases. There is more information about this in the how cancers grow section.

Cancers are named and treated according to where they started developing even if they have spread to other parts of the body. For example if you have lung cancer that has spread to the liver, it is a lung cancer with liver metastases or secondaries. It is not called liver cancer. This is because the cells in the liver are actually cancerous lung cells. They are not liver cells that have become cancerous.

 

Why can't anyone find the primary cancer?

Normally it is easy to find the primary cancer. Either it will cause symptoms or it will be seen on a scan. But sometimes secondary cancers are found in one or more parts of the body, but the doctor can't find the primary site. This is called cancer of unknown primary (CUP or UPC).

There are a number of reasons why the primary cancer cannot be found. It may be that

  • The secondary cancer has grown very quickly, whilst the primary cancer is still very small – very small primary cancers may not be seen on scans
  • Your immune system has successfully attacked the original primary cancer and it has disappeared, while the secondary cancers are still growing – this is not common, but it can happen
  • The primary cancer may have been sloughed off – this can happen in the digestive system for instance, where a small cancer may become detached from the wall of the bowel and is passed out of the body with faeces
 

How doctors know you have a cancer of unknown primary (CUP) 

The different types of cells in your body are named according to the body organ they belong to and also the job they do. When a cancer develops, it is named according to the type of cell it starts in. Doctors can often tell the type of cancer by what the cells look like under a microscope. For instance, cells from a lung tumour may look like breast cells that have become cancerous. So the doctor knows it is a breast cancer that has spread to the lung. 

Sometimes cancer cells don’t look like any particular type of normal cell. The cells are too primitive. They have not become specialised enough to look like breast cells or lung cells. Cells like this are known as poorly differentiated. When the cells are in such early stages it can make it very difficult for the doctor to tell what kind of cell the cancer started from. In this case, the cancer is called a poorly differentiated cancer of unknown primary.

Most cancers are cancers of the epithelial cells. Epithelial cells are found in tissues throughout the body. Cancers that start in epithelial tissue are called carcinomas. More than 85% of all cancers are carcinomas. Other types of cancers develop from different types of body cell. They include

  • Sarcomas, which develop from the cells muscles, fat, nerves or other soft tissues
  • Leukaemias, which are cancers of white blood cells found in the bone marrow and
  • Lymphomas, which are cancers of the cells of the immune system

There is information about different cell types and cancer in the about cancer section.

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Updated: 5 December 2012