Neuroendocrine neoplasms of the digestive system – current classification and terminology

Abstract

The system of classification and terminology of neuroendocrine neoplasms (NENs), updated in 2017 by AJCC and in 2019 by WHO, is now recommended for general use. This article is a review of this classification with respect to NENs of the digestive tract. Within the new system, two categories of neuroendocrine neoplasms of the digestive system were introdu­ced, differing in morphology, clinical course and treatment, as based on differentiation and histological maturity grading (G). Among NENs of the digestive tract, well differentiated neuroendocrine tumours, with Ki-67 proliferation index below 20%, NET G1 and NET G2, histologically resembling normal neuroendocrine cells were distinguished. Neuroendocrine neoplasms with Ki-67 above 20% – termed neuroendocrine carcinoma NEC (poorly differentiated carcinoma G3) – were found to be heterogeneous. In every organ of the digestive tract a limited group of well differentiated tumours with Ki-67 above 20%, but typically less than 55% (well differentiated high grade NET G3) was distinguished. The remaining poorly differentiated neuroendocrine neoplasms with Ki-67 above 20%, usually over 55%, were classified as NEC (high grade neuroendocrine carcinoma). Within NEC, two groups were distinguished – large cell and small cell carcinomas. By introducing this new classification based on clinical and molecular research, any confusion between NET G3 and NEC is avoided. NEC, goblet-cell carcinoid of the appendix and MiNEN, which should be classified according to criteria applied to adenocarcinomas of their respective organs of the digestive system, are not discussed.