Neuroendocrine tumours or NETs, is the umbrella term for a group of relatively uncommon cancers, often called the ‘quiet cancers’.  This is because NETs are often slow growing and so the symptoms can take time to develop, may be vague, or attributed to more common and less serious problems such as irritable bowel syndrome (IBS), Crohn’s disease, peptic ulcer disease or gastritis.

Neuroendocrine cancer is formed in the diffuse neuroendocrine system, which is made up of neuroendocrine cells found in the respiratory and digestive tracts. The respiratory tract includes the bronchial tubes and lungs. The digestive tract starts at the mouth and ends at the rectum.

Neuroendocrine cells are also found in the endocrine glands, such as the adrenal glands, pancreas, thyroid and pituitary. These cells are also found in the ovaries and the testes.

How NETs are formed is still not fully understood. As with all forms of cancer, NETs arise when cells multiply rapidly in the body. Normal cells in our body divide in a controlled manner, but in cancer the control signals go wrong. This causes abnormal cells to form, which divide quickly resulting in tumour growth.

Terminology of NETs

The terminology for NETs can be very confusing. The name for NETs has changed somewhat over the last few years. Carcinoid is sometimes still used, but the more accurate description is NET or Gastroenteropancreatic (GEP) NET. The term GEP came about because the tumours often arise in the cells of the stomach (gastro), intestines (entero) and the pancreas:

  • “G” – gastro (stomach)
  • “E” – entero (intestines)
  • “P” – pancreatic (pancreas)

Types of Neuroendocrine Tumours

There are a number of different types of NET. Although they all arise from neuroendocrine cells and share certain similar biological features, they each have a slightly different way of presenting themselves, both in terms of symptoms and how they look under a microscope.


  • Carcinoids: lung, thymic, gastric, duodenal, pancreatic, small intestine, appendiceal, colon, rectal, ovarian and carcinoid tumours of unknown origin (unknown primary)
  • Functioning and Non-Functioning Pancreatic Tumours
  • Multiple Endocrine Neoplasias (MEN)
  • Gastrinomas
  • Insulinomas
  • Glucagonomas
  • Phaeochromocytomas
  • VIPomas
  • Goblet Cell Carcinoids

The place where the NET first appears is called the ‘primary’ site. However, the NET may spread and be found in other parts of the body e.g. the liver, and if this occurs your doctor may refer to these as a ‘secondary’ tumour or metastases.

Carcinoid NETs are the most common and typically account
for about 55% of all NETs diagnosed

Some NETs produce abnormally large amounts of hormones, which are chemicals made by the body to trigger its various functions.  For example most carcinoid tumours produce excess serotonin and gastrinomas produce an excess of gastrin. These NETs have a related syndrome which means that the hormones secreted cause noticeable symptoms like flushing, diarrhoea, cramps, wheezing, heart valve damage and skin changes.

Most NETs occur without any genetic link, but there are cases where they are present as part of a familial endocrine cancer syndrome such as MEN1, MEN 2, neurofibromatosis type 1 or Von Hippel Lindau.