Cancer of unknown primary (CUP)
Your body is made up of billions of cells that can only be seen under a microscope. The cells group together to make up the tissues and organs of our bodies.
Usually, we have just the right number of each type of cell. This is because cells produce signals to control how much and how often the cells divide. If any of these signals are faulty or missing, cells might start to grow and multiply too much and form a lump called 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.
Read more about how cancer starts
The primary cancer is the place where a cancer starts growing. 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 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 cancerous bowel cells. They are not liver cells that have become cancerous. Doctors confirm this by looking at the cells under the microscope.
Read more about how cancers grow and spread
It's normally straightforward to find the primary cancer. Usually, people go to their GP because of symptoms. The GP might then refer you to a specialist doctor and for tests that confirm where the cancer started (the primary tumour).
Sometimes secondary cancers are found in one or more parts of the body, but despite several tests, doctors can't find the primary tumour. If tests show that you have a cancer that has spread, but your doctors can’t find the primary tumour, it’s called a malignancy of unknown primary origin (MUO).
You then have some initial tests to try to find out the primary cancer. If the primary cancer doesn’t show up on the initial tests, your doctor may refer you to a team of doctors who specialise in cancer of unknown primary (CUP).
The CUP specialist team may arrange for more tests to search for the primary cancer. You have a confirmed cancer of unknown primary (cCUP) if they can't find the primary cancer after the specialised tests.
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)
Find out about lymph nodes and where they are in your body
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 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.
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
Read more about different cell types and cancer
Around 8,100 people are diagnosed with cancer of unknown primary in the UK each year. That's 2 out of every 100 (2%) cancers diagnosed.
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.
More information about the risks and causes of cancer of unknown primary
Symptoms of CUP depend on where the cancer has spread to in your body. Possible symptoms of CUP include:
weight loss
sickness and loss of appetite
weakness or feeling very tired
pain
breathlessness
a cough that won't go away
swollen lymph nodes
Last reviewed: 13 May 2024
Next review due: 13 May 2027
There is no staging system for cancer of unknown primary (CUP). Doctors can group CUP by the type of cell the cancer started in.
Most people are diagnosed with CUP after they become unwell and go to A&E. Others start by seeing their GP. Find out what to expect when you see a GP and the tests you might have.
Treatment for CUP depends on a number of things, including where the cancer is and your general health. You might have a targeted cancer drug or chemotherapy. Sometimes you may have surgery or radiotherapy to help control your symptoms.
Advice and support is available to help you cope with cancer of unknown primary (CUP) and its treatment.
Survival for cancer of unknown primary depends on many different things such as the type and stage of cancer. No one can tell you exactly how long you will live.
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