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Demystifying Neuroendocrine Neoplasms (NENs) and Their Classification

As neuroendocrine cells are distributed throughout the body, neuroendocrine neoplasms (NENs) have been found in the central nervous system, respiratory tract, larynx, gastrointestinal tract, thyroid, skin, breast, and urogenital system. In addition, they are highly variable in morphology, genomic alterations, clinical manifestation and outcomes. Most NENs are classified as well-differentiated neuroendocrine tumors (NETs), comprising 80-90% of all diagnosed NENs. The rarer form of NENs termed Neuroendocrine carcinomas (NECs) are poorly differentiated and more aggressive (Pavel et al. 2020) (Table 1).

Radiopharmaceuticals for diverse neuroendocrine neoplasms.
Table 1: The WHO/IARC universal taxonomy for epithelial NENs (Rindi G et al. 2022)

The most frequent sites of origin of the neoplasms are the digestive and pancreatic systems (70%), followed by the respiratory tract (25%) (Klöppel G. 2017). Epithelial well-differentiated neoplasms of the digestive and pancreatic systems, comprising tumor cells that retain the morphological and molecular features of their precursor cells, are defined as gastroenteropancreatic neuroendocrine tumors (GEP-NETs) (Rindi G et al. 2022).


  • Pavel, M. et al. 2020. “Gastroenteropancreatic Neuroendocrine Neoplasms: ESMO Clinical Practice Guidelines for Diagnosis, Treatment and Follow-Up.” Annals of Oncology 31(7): 844–60. DOI: 10.1016/j.annonc.2020.03.304

  • Klöppel, Günter. 2017. “Neuroendocrine Neoplasms: Dichotomy, Origin and Classifications.” Visceral Medicine 33(5): 324–30. DOI: 10.1159/000481390

  • Rindi, Guido et al. 2022. “Overview of the 2022 WHO Classification of Neuroendocrine Neoplasms.” Endocrine Pathology 33(1): 115–54. DOI: 10.1007/s12022-022-09708-2