Cancer of unknown primary (CUP) and cell types

Cancers are made up of different types of cells. Knowing the type of cell can give the doctors a better idea about where the cancer may have started.

Cancer of unknown primary (CUP) and cell types

For most secondary cancer diagnoses, doctors can find out where the cancer cells have spread from by looking at them under a microscope. For example, cells from a primary breast cancer that have spread to the liver usually still look like breast cancer cells. With cancer of unknown primary, doctors cannot always tell which part of the body the cancer cells are from.

But cancers are made up of different types of cells. These can be grouped based on the type of cells they are made up of. Knowing the type of cell can give the doctors a better idea about where the cancer may have started. This helps them to plan the best treatment.

Carcinomas are the most common type of cancer cell. About 85% of all cancers in the UK are carcinomas. They are cancer cells that start in the epithelial tissues. The epithelial tissues cover or line all the organs in the body. Our skin and the linings of cavities inside the body, such as the chest cavity, are also made of epithelial cells.

There are a few different types of carcinomas.


These cancers grow from gland cells that line or cover some organs in the body. Gland cells make substances that the body needs, such as hormones. Adenocarcinomas are the most common type of cancer cell found in people with CUP.

Adenocarcinomas usually start in one of the following areas:

  • breasts
  • lungs
  • bowel
  • pancreas
  • kidneys
  • liver
  • stomach
  • ovaries
  • womb.

Squamous cell carcinomas

These cancers develop from flat cells that are usually found on the surface of the skin or in the inner lining of organs. They can develop in organs such as the lungs, gullet (oesophagus), cervix and the head and neck area.

Neuroendocrine carcinomas

These are a rarer type of cancer. They are made up of nerve cells that produce hormones. We have more information about neuroendocrine tumours nets.

Poorly or undifferentiated cancer cells

Poorly differentiated cancer cells look very different from normal cells.

Undifferentiated cancer cells look extremely different to normal cells.

Both poorly and undifferentiated cancer cells make it difficult to tell the type of primary cancer. Further tests may sometimes tell the doctors more.

About our information

  • References

    Below is a sample of the sources used in our chondrosarcoma information. If you would like more information about the sources we use, please contact us at

    Fizazi K et al. Cancers of unknown primary site. ESMO clinical practice guidelines for diagnosis, treatment and follow up. 2015. Annals of oncology 26 (supplement 5): v133-v138.

    National Institute for Health and Care Excellence (NICE). Metastatic malignant disease of unknown primary origin in adults: diagnosis and management. 2010.

    UK Oncology Nursing Society (UKONS). Acute oncology initial management guidelines. [Internet], 2018. Available from: [accessed March 2022].

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Chief Medical Editor, Professor Tim Iveson, Consultant Medical Oncologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

Date reviewed

Reviewed: 01 December 2022
Next review: 01 December 2025
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.