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Types of cancer of unknown primary (CUP)

Men and woman discussing unknown primary cancer

This page is about the different types of CUP. There is information about


A quick guide to what's on this page

Types of unknown primary tumour

When the primary tumour cannot be found, important clues to the type of cancer are whether you are male or female and where the secondary tumour is found in the body. A tissue sample (biopsy) and tumour markers may also give more information about the primary cancer tumour. 

Most cancers of unknown primary are adenocarcinomas (a type of carcinoma) and they make up between 60% and 90%. But there are other less common types and they include those below.


In some cases of melanoma skin cancers the primary tumour is not found. 


Lymphoma can be mistaken for cancer of unknown primary (CUP). Tumour markers can help to make a diagnosis of lymphoma.

Rare types of carcinoma

A very small number of carcinomas of unknown primary may be germ cell tumours. Between 5 and 10% may be squamous cell tumours and about 5% are neuroendocrine tumours.


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Different types of unknown primary tumours

Even when a primary tumour cannot be found, better diagnostic tests may now give much more information about it. Your gender, the position of the secondary cancer in the body, and detailed laboratory information about the tumour cells, are important clues in your diagnosis. The type of cancer that you have depends on the types of cell it has developed from. To find this out, your doctor will take a tissue sample (biopsy) and send it to a laboratory where a pathologist will examine it closely.

The tumour cells may look very abnormal under a microscope. The term for this is poorly differentiated or undifferentiated. This means that it may not be possible to tell what type of cancer it is just by looking at the shape and structure of the cells.

The pathologist may also test the tissue sample using antibodies to find tumour markers. Tumour markers are chemicals produced by cancer cells. Some tumour markers are produced by one type of cancer, while others can be made by several different types of cancer. The study of tumour markers is called immunohistochemistry (IHC) and tumour markers are often called IHC markers.



In some cases of melanoma the primary tumour cannot be found. When a biopsy of the secondary tumour shows that it is melanoma it is called a melanoma of unknown primary (MUP). It is then treated as stage 3 or 4 melanoma.



Some lymphoma cells can look like other types of cancer. Lymphoma can sometimes be mistaken for secondary cancer in the lymph nodes and is classed as CUP. Tumour markers can help to make a diagnosis of lymphoma. This means that lymphoma is the primary cancer and it is treated as any other lymphoma of a similar type and stage. Very rarely a mass of leukaemia cells can also look similar to other cancers.



Most cancers of unknown primary (CUP) are carcinomas.  They are subdivided into

Germ cell tumours

Germ cell tumours develop from cells that become sperm or eggs. Almost all of them are either seminoma or teratoma of the testicle. Germ cell tumours occur very rarely in women. A very small number of ovarian tumours are germ cell and are usually found in young women. Rarely, these tumours occur in other parts of the body, which can make the primary tumour very hard to find. Immunohistochemistry tests (IHC) can identify this type of tumour. The secondary tumour often occurs in lymph nodes in the middle area of the abdomen or chest.

Squamous cell tumours

Squamous cell tumours make up 5 to 10% of CUP. Squamous cells are found in the skin and in the membranes that line the airways close to the outside of the body, such as the nose, throat, cervix and anus. The secondary tumours are usually noticed as enlarged lymph nodes in the neck or groin. Local treatments such as surgery and radiotherapy can work well for these tumours.

Neuroendocrine tumours

Neuroendocrine tumours (NETS) make up about 5% of CUP. The most common of these are carcinoid tumours which occur mainly in the small bowel or other parts of the digestive system. NETS may also occur in the lung, pancreas, kidney, ovary and testicle. If this type of CUP can be identified, it can respond very well to treatment.


Adenocarcinoma makes up at least 3 out of 5 (60%) of all unknown primary cancers. However the true figure may be higher than this. Adenocarcinoma is cancer of glandular tissue. The primary tumour occurs in the tubes or ducts of body organs. Studies suggest that in unknown primary adenocarcinoma the secondary cancers are found in the

  • Lung
  • Pancreas
  • Bowel
  • Kidney or adrenal gland
  • Liver or bile duct
  • Stomach 
  • Ovary or uterus
  • Prostate

A very small number of unknown primary adenocarcinomas can occur in the breast.

Carcinoma of solid tumours

The primary site of carcinoma of solid tumours is usually the liver, kidney or endocrine glands. They are often grouped with adenocarcinomas.

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Updated: 15 July 2014