Neuroendocrine carcinomas (NEC) are rare in the gastrointestinal (GI) tract, whereas they are frequent in the form of small cell carcinoma (SCLC) in the lung. Therefore, most of the suggested guidelines arise from analogy to SCLC.
As for other extrapulmonary primary tumor locations, published data on NEC of the GI tract are scarce. This guideline encompasses all WHO grade 3 (G3) gastroenteropancreatic (GEP) neoplasms; however, in the future, G3 neuroendocrine neoplasms (NEN) will probably be separated according to differentiation, as explained below, and potentially according to organ of origin, such as for well-differentiated neuroendocrine tumors (NET) G1/G2.