Cancer of unknown primary

Tuesday 1 May, 2018

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On this page: What is cancer of unknown primary? | How can doctors tell it is a secondary cancer? | Why can’t the primary cancer be found? | Does it matter that the primary cancer can’t be found? | Will I need lots of tests? | What are the risk factors? | What are the symptoms? | How common is CUP? | What are the different types of CUP? | Key points


For an overview of what to expect during all stages of your cancer care, visit What to expect – Cancer of unknown primary. This is a short guide to what is recommended, from diagnosis to treatment and beyond.

What is cancer of unknown primary?

This is a secondary cancer that has spread to a new location from somewhere else in the body. It's not clear where in the body it started, so the primary cancer is unknown.

For most people diagnosed with cancer, the primary cancer is easy to identify. Doctors conduct tests to find out where the cancer started to grow and to see if it has spread.

When cancer is found in one or more secondary sites but it is not clear from the test results where the cancer began, the cancer is usually known as cancer of unknown primary, or CUP. Health professionals may also call it metastasis of unknown origin (MUO), metastatic malignancy of unknown primary, or occult primary cancer.

Because it is a secondary cancer, CUP is considered advanced cancer. This means that in some cases, treatment focuses on controlling symptoms. In other cases, doctors actively treat the cancer while still making sure that symptoms are well managed. Your doctor will discuss the best approach for you.

It is a shock to be told you have any type of advanced cancer. For information on coping with the diagnosis, see Living with Advanced Cancer.

How can doctors tell it is a secondary cancer?

To diagnose secondary cancer, a specialist doctor called a pathologist examines the cancer cells under a microscope. The pathologist can see that the cancer cells do not belong to or originate in the surrounding tissue, and this can be confirmed by further laboratory tests. With CUP, the cancer cells could possibly have come from a number of places but no primary cancer can be found.

Why can't the primary cancer be found?

There could be several reasons why the primary cancer cannot be found. It may be that:

  • the secondary cancer has grown and spread quickly, but the primary cancer is still too small to be seen on scans
  • your immune system has destroyed the primary cancer, but not the secondary cancer
  • the primary cancer cannot be seen on x-rays, scans or endoscopies because it is hidden by a secondary cancer that has grown close to it
  • the primary cancer was removed during surgery for another condition without doctors realising the cancer was there, so some cancer cells remained behind and have since spread
  • there may never have been a detectable primary cancer mass – a single cell may have changed into cancerous cells and then spread through the body.

Does it matter that the primary cancer can't be found?

Finding the primary cancer helps doctors decide what treatment to recommend. If it can't be found, the treatment path can be less clear but treatment can still be given. To help plan the treatment, your doctor will try to learn as much as possible about the spread of the cancer, the cells involved, your symptoms and your medical history.

Will I need lots of tests?

Many people find they need several tests to try to find where the cancer started. The tests may take time and be tiring, particularly if you are feeling unwell. You may also feel frustrated if the tests don't find the primary cancer.

Your doctors will only suggest tests they feel are necessary. It is okay to ask them to explain the tests and the difference the results will make to your care. With this information, you can weigh up the potential benefits of the tests and make an informed decision about whether to have them. At some point, your doctors may decide that having more tests won't help find the primary cancer and it may be of more benefit to focus on your treatment.

If you do decide against having further tests, you may find your family and friends encourage you to continue. This can be a challenging situation, and it may help to explain your reasoning to them.

What are the risk factors?

Different cases of CUP will have different causes, but without knowing where the cancer started, it's difficult to be specific. Cancer is a group of more than 200 different diseases. Each type of cancer has different risk factors, such as getting older, eating an unhealthy diet, smoking, drinking too much alcohol, being overweight or certain infections. These risk factors may play a role in some cases of CUP.

What are the symptoms?

Symptoms are different for everyone and are related to the area where the secondary cancer is found. Some people with CUP have few or no symptoms; others have a range of symptoms that may include:

  • shortness of breath cough
  • bone pain and/or back pain
  • swelling and discomfort in the abdomen, feeling sick (nausea) or fluid collecting in the abdomen (ascites) yellowing of the skin and eyes (jaundice)
  • swollen lymph nodes in the neck, underarm, chest or groin
  • looking pale, feeling tired and becoming breathless due to a lack of red blood cells (anaemia).

You may also have general symptoms such as unexplained weight loss, poor appetite, night sweats or fatigue. Not everyone with the symptoms listed above will have CUP, but see your general practitioner (GP) if you are concerned.

How common is CUP?

CUP is the 11th most common cancer in Australian women and the 12th most common cancer in Australian men. 3 There are over 2500 new cases of CUP diagnosed each year in Australia. For both women and men, CUP is more likely to occur over the age of 60. 4

What are the different types of CUP?

Even if tests can't find where the cancer started, your doctor will try to work out what type of cell the cancer developed from. Knowing the type of cell helps doctors work out what sort of treatment is most likely to be helpful.

Most cancers are cancers of the epithelial cells. These are found in the cells lining the skin and internal organs. Cancers that start in epithelial tissue are called carcinomas. In most people with CUP, doctors can tell that they have some sort of carcinoma. There are different types of carcinoma depending on which type of epithelial cell is affected (see below for more information).

Types of CUP

Adenocarcinoma
  • Adenocarcinoma develops from glandular cells, which form part of the lining of many organs.
  • Common primary sites include the breast, colon, prostate, stomach, pancreas, liver and lungs.
  • Adenocarcinoma makes up about 50% of CUP.
Poorly differentiated carcinoma
  • This means there is enough detail to tell that the cells are a carcinoma, but there is not enough detail to work out the type of cancer.
  • About 30% of people with CUP have a poorly differentiated carcinoma.
Squamous cell carcinoma (SCC)
  • This carcinoma develops from squamous cells, which are thin, flat cells normally found on the surface of the skin or the lining of certain organs.
  • Common primary sites include the head and neck area, skin, oesophagus, lungs, vagina and cervix.
  • SCC makes up about 10–15% of CUP.
Undifferentiated neoplasm
  • Neoplasm is another word for tumour. In an undifferentiated neoplasm, the cells can be identified as cancerous, but it is not possible to work out whether they are a carcinoma, sarcoma, melanoma or another form of cancer.
  • About 5% of people with CUP have an undifferentiated neoplasm.
Neuroendocrine carcinoma
  • This carcinoma develops from specialised nerve cells that sometimes produce hormones.
  • Common primary sites include the pancreas and gastrointestinal tract.
  • About 3% of people with CUP have a neuroendocrine carcinoma.

Key points

  • CUP, or cancer of unknown primary, is a secondary cancer that has spread from somewhere else in the body, but it's not clear where in the body it started. The secondary cancer can be identified, but not the primary.
  • It is also called metastasis of unknown origin (MUO), metastatic malignancy of unknown primary, or occult primary cancer.
  • There are a number of reasons why it may not be possible to find the primary cancer. For example, the primary tumour might be too small to be seen on scans, it might be hidden by secondary tumours that have grown nearby, or it may have been destroyed by the immune system.
  • Doctors will try to find the primary cancer because it helps them decide what treatment to recommend.
  • Your doctor may ask you to have several tests to try to find the primary cancer.
  • If the tests cannot find where the cancer started, the doctor will try to identify the type of cell the cancer developed from. This gives the doctor a better idea of where the cancer may have started.
  • Some people with CUP feel too unwell or simply prefer not to have lots of tests.
  • Symptoms of CUP are often related to the area where the secondary cancer is found, and may include shortness of breath, bone pain and/or back pain, swelling and discomfort in the abdomen, and swollen lymph nodes in the neck, underarm, chest or groin.
  • CUP is the 11th most common cancer in Australian women and the 12th most common cancer in Australian men.

Reviewed by: A/Prof Linda Mileshkin, Medical Oncologist, Peter MacCallum Cancer Centre, VIC; Karen Hall, Clinical Nurse, Oncology/Haematology, Flinders Medical Centre, SA; Rebecca James, 13 11 20 Consultant, Cancer Council SA; Prof Chris Karapetis, Network Clinical Director (Cancer Services), Southern Adelaide Local Health Network, Head, Department of Medical Oncology, Director, Clinical Research in Medical Oncology, Senior Consultant, Southern Oncology SA, Flinders Private Hospital, Flinders Medical Centre and Flinders University, SA; Frank Stross, Consumer.

3. Australian Institute of Health and Welfare (AIHW), Cancer in Australia 2017, AIHW, Canberra, 2017.

4. Australian Institute of Health and Welfare (AIHW), Australian Cancer Incidence and Mortality (ACIM) books: cancer of unknown primary site, AIHW, Canberra, December 2017.

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