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 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.

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.