What are neuroendocrine tumours (NETs)?

Neuroendocrine tumours (NETs) are rare tumours that start in neuroendocrine cells. You might also hear these tumours called neuroendocrine neoplasms (NENs) or neuroendocrine carcinomas (NECs). 

There are a number of different types of NETs. The type you have depends on the type of cell that the cancer started in. 

Most NETs develop slowly over some years. They may not cause symptoms in the early stages. It’s not unusual for people to find that a NET has already spread to another part of the body when they are diagnosed. 

The neuroendocrine system

The neuroendocrine system is made up of nerves and gland cells. It makes hormones and releases them into the bloodstream.

Neuro means nerve and endocrine refers to the cells of the endocrine system. The endocrine system is a network of glands and organs in the body that make hormones. It is also called the hormone system.

There are neuroendocrine cells in most organs of the body, including the:

• food pipe (oesophagus)
• stomach
• lungs
• small and large bowel
• pancreas
• liver

What do neuroendocrine cells do?

Neuroendocrine cells have different functions depending on where they are in the body. They control how our bodies work. This includes our growth and development, how we respond to changes such as stress, and many other things. 

For example, neuroendocrine cells of the lung make hormones that control the flow of air and blood in the lungs. And neuroendocrine cells of the gut (digestive system) make hormones to control:

• the production of digestive juices

• the muscles that move food through the bowel

Where NETs start

NETs can start in different parts of the body. Like all cancers, NETs are named after the place they start growing. For example, a NET that starts in the lung is called a lung NET. This is the primary cancer. If the cancer spreads to another part of the body, it’s called secondary cancer. 

Around 5 out of every 10 NETs (50%) start in the digestive system. This is also called the gastrointestinal (GI) system. It includes the:

• stomach
• small and large bowel
• pancreas
• back passage (rectum)

Around 2 out of every 10 NETs (20%) start in the lung. NETs can also start in other places such as the:

• food pipe (oesophagus)
• appendix
• skin
• prostate
• womb
• adrenal, parathyroid and pituitary glands

In some cases, doctors do not know where the original NET started. This is called cancer of unknown primary (CUP).

Cancerous or non cancerous

There is some debate among doctors about how NETs should be grouped and what they should be called. NETs develop in different parts of the body and behave in different ways. For example, some NETs grow slowly while others are faster growing. 

All NETs are malignant (a cancer) by definition. Some NETs are diagnosed early and you might be able to have treatment to cure it. 

More recently doctors have been calling them neuroendocrine neoplasms (NENs). This is an umbrella term for this group of disorders. Then depending on how slow or fast growing the cells are they are called either neuroendocrine tumours (NETs) or neuroendocrine carcinomas (NECs).

NETs are usually slower growing and the cell changes are called well-differentiated. NECs tend to be faster growing and the cell changes are called poorly-differentiated.

The wording can be quite confusing. Talk to your doctor if you are not sure. 

How common are NETs?

NETs are rare. Over 4,000 people are diagnosed with a NET each year in the UK. They are getting more common across the world. This might be because there are better tests to diagnose them. 

NETs can develop at any age, including in children. But the average age of diagnosis is around 50 to 60 years old.