Expansion of the world health organization (WHO) gastric neuroendocrine tumor classification: introducing types 4 and 5.

The 6th Edition of the World Health Organization Classification of Digestive System Tumours has introduced two new categories of gastric neuroendocrine tumors, reflecting recent data on subtypes that do not conform to the three established subtypes: type 1 (associated with autoimmune gastritis), type 2 (associated with Zollinger-Ellison Syndrome) and type 3 (sporadic). The new type 4 category is exceptionally rare and is caused by endogenous dysfunction of parietal cell proton pumps, potentially secondary to mutation of ATP4A. Type 5 gastric NETs are caused by chronic pharmacologic inhibition of the parietal cell proton pump, typically due to proton pump inhibitor use, and are increasingly common. Given the relatively low malignant potential of the new subtypes compared to sporadic (type 3) gastric NETs, it is important that clinicians and pathologists recognize these entities, limiting radical surgical resections when unnecessary.