Types of neuroendocrine tumours (NETs)

Neuroendocrine tumours (NETs) start in cells of the neuroendocrine system. Neuroendocrine cells are spread throughout the body. They receive messages (signals) from the nervous system and respond by making and releasing hormones. These hormones control many body functions, such as digestion and breathing.

NETs are classified as functional or non-functional. Functional tumours make too much of a certain hormone and cause symptoms. Non-functional tumours may make hormones but don’t cause any symptoms.

NETs can be indolent or aggressive. Indolent means the tumour grows slowly. Aggressive NETs grow quickly and tend to spread to other parts of the body.

NETs are grouped by where the tumour started in the body.

Gastrointestinal neuroendocrine tumours (GI NETs) are a common type of neuroendocrine tumour. They develop in organs of the gastrointestinal (GI) tract, including the small intestine, rectum, stomach, colon, esophagus and appendix. Most GI NETs grow slowly.

The most common GI NETs start in the small intestine and release serotonin. Too much serotonin leads to carcinoid syndrome, which is a group of symptoms caused by large amounts of serotonin and other chemicals released into the blood.

GI NETs used to be called carcinoid tumours. Carcinoid means carcinoma-like, but this does not accurately describe all GI NETs. Also carcinoid tumours are often confused with carcinoid syndrome, but GI NETs do not always make serotonin and cause carcinoid syndrome. For these reasons, the term carcinoid is no longer used to describe these tumours.

GI NETs are classified based on how different the cells are from normal cells (differentiation) and how fast the cells are growing (grade).

Well-differentiated GI NETs are made of cells that look and act much like normal cells. These tumours can be indolent, grow more quickly or be aggressive, so well-differentiated NETs are also given a grade. Find out more about grading for neuroendocrine tumours.

Poorly differentiated GI neuroendocrine carcinomas are aggressive cancerous tumours. They are made of cells that are very abnormal. These cells grow and divide very quickly compared to normal cells.

Lung neuroendocrine tumours (lung NETs) are a common type of neuroendocrine tumour. They develop in the lungs or airways, often in the airways that branch off from the windpipe into the lungs (bronchi).

There are 4 subtypes of lung NETs:

Typical carcinoid tumours are indolent tumours of the lung. The cells look and act much like normal cells. Typical carcinoid tumours tend to grow slowly.

Atypical carcinoid tumours are well-differentiated cancerous tumours of the lung. The cancer cells look and act much like normal cells. Atypical carcinoid tumours tend to grow slowly, but they may spread to other parts of the body.

Large cell lung neuroendocrine carcinomas are poorly differentiated cancerous tumours. This means the cancer cells are very abnormal. Large cell neuroendocrine tumours tend to be aggressive tumours that grow quickly. They are more likely to spread to other parts of the body.

Small cell lung neuroendocrine carcinomas, or small cell lung cancers, are also poorly differentiated cancerous tumours. They are aggressive tumours that grow quickly and often spread to other parts of the body.

Pancreatic neuroendocrine tumours (pNETs) are tumours that look like the islet cells of the pancreas. Islet cells form clusters that include different types of cells that make hormones, including alpha cells (A cells) that make glucagon, beta cells (B cells) that make insulin and delta cells (D cells) that make somatostatin.

pNETs are classified based on how different the cells are from normal cells and how fast the cells are growing.

Well-differentiated pNETs are made of cells that look and act much like normal cells. They tend to be indolent tumours that grow slowly.

Poorly differentiated pancreatic neuroendocrine carcinomas are cancerous tumours with very abnormal cells. They tend to be aggressive tumours that grow quickly and are usually advanced when diagnosed.

The naming of pNETs may include the type of cell it looks like or the hormone it makes, such as beta cell insulin-producing NET.

Functional pNETs cause symptoms from the hormones that are released. They are often named based on the type of hormone they produce, such as:

• gastrinoma
• insulinoma
• glucagonoma
• somatostatinoma
• VIPoma
• ACTHoma

Since neuroendocrine cells are spread throughout the body, NETs can develop in many different places, including in endocrine glands. The following are also NETs:

• medullary carcinoma – a type of thyroid cancer that starts in the C cells of the thyroid
• parathyroid cancer or parathyroid adenoma
• thymic neuroendocrine cancer
• pheochromocytoma – starts in the chromaffin cells of the adrenal glands
• paraganglioma – starts in the chromaffin cells outside the adrenal glands
• pituitary gland tumours
• neuroendocrine tumours of the ovaries or testicles
• Merkel cell carcinoma – a type of non-melanoma skin cancer