The ENETS 2023 guideline for gNETs are combined with the guidelines for Duodenal NET (dNET) due to their close relationship in anatomical terms. 

Gastric neuroendocrine neoplasms (gNENs) are tumours with an increasing annual incidence and.  The vast majority of gNENs are well-differentiated neuroendrocrine tumours (NETs), which are usually classified according to the background gastric pathology into three major categories known as “types” – these should not be confused with “grades” which are generated by histopathological tissue sampling (biopsy). 

Type I when chronic atrophic gastritis (CAG) is present resulting in hypergastrinaemia (high Gastrin) and high gastric pH. They are the most common type, accounting for 75%–80% of all gNENs. Type I gNETs are normally ‘indolent’ with negligible risk of metastases (<5%) and excellent long-term survival (almost 100%). 

Type II when the tumour occurs due to hypergastrinaemia (high Gastrin) but low gastric pH and in the context of Zollinger–Ellison syndrome (ZES) via a Gastrinoma.  They most often associated with multiple endocrine neoplasia type I (MEN-I) syndrome.  Sporadic Type II is rare. This type accounts for 5% of gNENs.  

Type III, which are sporadic lesions not associated with hypergastrinaemia and with normal gastric pH (15%–25% of gNENs).  Type III tumours are usually more aggressive, owing to the high prevalence of metastatic disease (>50%) and the more unfavourable long-term survival (5-year survival rate 70%), although they present well-differentiated morphology in the majority of cases. Type III gNENs may also have higher grades.