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Hero banner. Neuroendocrine tumours (NETs).

What you need to know

Diagram of the abdomen and upper body showing the anatomical classification of NETs.

Adapted from Canadian Cancer Society.1

Definition and distribution of NETs

NETs are a group of neoplasms that originate in secretory cells known as neuroendocrine cells, which are distributed throughout the body.1,2

They are located in three broad areas:

  1. Isolated neuroendocrine cells, scattered throughout most tissues1,3

  2. Aggregates of neuroendocrine cells in organs3

  3. Classic endocrine glands1

Diagram of the abdomen and upper body showing the anatomical classification of NETs.

Adapted from Oronsky B, et al. 2017.5

Anatomical classification of NETs

NETs are classified according to their anatomical site of origin, with the vast majority arising in the gastroenteropancreatic (GEP) or bronchopulmonary (BP) tracts (GEP-NETs and BP-NETs, respectively).4

NETs are traditionally subclassified according to the embryological origin of their site:5

  • Foregut

  • Midgut

  • Hindgut

The nature of NETs

 

Not only do NETs differ in their anatomical site, they also vary in their level of differentiation and ‘aggressiveness’ (grading).4,6

Differentiation

  • Refers to the extent to which tumour cells morphologically resemble healthy cells from the same tissue4,6

  • By definition, NETs are well-differentiated7

  • NETs are usually organised into well-developed architectural patterns5,8

  • The invasive and metastatic potential of NETs is variable7

Grading

  • NETs are given a histological grade which refers to the aggressiveness of the tumour, i.e., G1, G2 or G34

  • Higher-grade tumours are associated with negative patient outcomes8,9

  • Well-differentiated NETs are nearly always G1 or G2 (though some can be G3); poorly differentiated tumours are referred to as neuroendocrine carcinomas (NECs) and are always G34,7

Functioning and non-functioning NETs2,4,10

 

In the case of NETs, increased proliferation of neuroendocrine cells sometimes leads to hypersecretion of hormones.4 When NETs cause clinical symptoms due to hormone hypersecretion, they are described as ‘functioning’. However, most NETs do not produce biologically active hormones and are termed ‘non-functioning’.4

Type of functioning NET

Hormone(s) secreted

Anatomical site

Functioning carcinoid

Serotonin, tachykinin, prostaglandins, 5-hydroxytryptophan, histamine

Small intestine, lung or pancreas

ACTHoma

Adrenocorticotropic hormone (ACTH)

Pancreas, bronchus, thymus

Insulinoma

Insulin

Pancreas

Gastrinoma

Gastrin

Gastrinoma triangle

Glucagonoma

Glucagon

Pancreas

PPoma

Pancreatic polypeptide (PP)

Pancreas

Somatostatinoma

Somatostatin

Pancreas/duodenum

VIPoma

Vasoactive intestinal peptide (VIP)

GEP tract, adrenal gland

Incidence of NETs

 

Historically considered as rare tumours, NETs are becoming increasingly common.11 NET incidence has increased >500% in the last three decades. Approximately 6 in every 100,000 people will develop a NET.11

In the UK:

  • The age-adjusted incidence of GEP-NETs increased 3.8- to 4.8-fold from 1973 to 200712

  • In England specifically, neuroendocrine neoplasm (NEN) incidence rose to 9.37 per 100,000 in 2016, remaining at a similar rate in 201713

Graphic showing Incidence of NETs in the UK and England specifically.

Adapted from White BE, et al. 2019.13

Symptoms of NETs

 

The majority of NETs are non-functioning and symptoms, if they do occur, tend to be vague and non-specific.4,5

 

Symptoms associated with BP-NETs

 

  • BP-NETs are associated with wheezing, coughing, haemoptysis, dyspnoea, chest pain and recurrent pneumonia4,5,14

  • Peripherally located lung NETs are usually asymptomatic5

 

Symptoms associated with GEP-NETs

 

  • The most prominent symptoms are abdominal pain and change in bowel habit (due to mass effects of the tumour causing intermittent bowel obstruction)4,14

  • Fatigue is particularly prevalent in those with pancreatic NETs14

ACTH, adrenocorticotropic hormone; BP, bronchopulmonary; GEP, gastroenteropancreatic; GI, gastrointestinal; NEC, neuroendocrine carcinoma; NEN, neuroendocrine neoplasm; NET, neuroendocrine tumour; PP, pancreatic polypeptide; VIP, vasoactive intestinal peptide.

References

  1. Canadian Cancer Society. The neuroendocrine system. Available at: https://cancer.ca/en/cancer-information/cancer-types/neuroendocrine/what-is-neuroendocrine-cancer/the-neuroendocrine-system [Accessed October 2025].

  2. Kidd M, et al. Cell Mol Gastroenterol Hepatol 2015;1:131–153.

  3. Ameri P & Ferone D. Neuroendocrinology 2012;95:267–276.

  4. Raphael M, et al. CMAJ 2017;189:E398–E404.

  5. Oronsky B, et al. Neoplasia 2017;19:991–1002.

  6. Chung C. Am J Health Syst Phar 2016;73:1729–1744.

  7. Rindi G, et al. Mod Pathol 2018;31:1770–1786.

  8. Cavalcanti M, et al. Int J Endocr Oncol 2016;3:203–219.

  9. Dasari A, et al. JAMA Oncol 2017;3:1335–1342.

  10. Asha H, et al. Indian J Endocrinol Metab 2011;15:346–348.

  11. Frilling A, et al. Endocr Relat Cancer 2012;19:R163–R185.

  12. Fraenkel M, et al. Endocr Relat Cancer 2014;21:R153–R163.

  13. White BE, et al. Endocr Abstr 2019; 68:OC3.

  14. Basuroy R, et al. Neuroendocrinology 2018;107:42–49.

UK | October 2025 | FA-11462546

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