Mixed adenoma-neuroendocrine tumor of the stomach: analysis of nine cases with literature review.

Mixed adenoma-neuroendocrine tumor (MANET) comprises adenoma and well-differentiated neuroendocrine tumor (NET) components. Given the limited information on this due to its rarity, we aimed to clarify the clinicopathologic features and optimal management of gastric MANETs in a case series and literature review. Nine patients with gastric MANETs, including eight male and one female patient (mean age, 72 years), were identified from the institutional pathology archive. Endoscopically, the tumors appeared as flat elevated lesions with sizes ranging from 0.8 to 4.4 cm. One patient had familial adenomatous polyposis, and no patient had autoimmune gastritis. All MANETs developed in the gastric body mucosa exhibiting chronic metaplastic atrophic gastritis. The glandular components were intestinal-type low-grade adenoma, and focal high-grade dysplasia was also recognized in three cases. The NET component was in middle/deep lamina propria in six cases and confined to deep lamina propria in the remaining three cases. Minimal cytologic atypia was found in the NET component, with no recognizable mitosis and a Ki-67 labeling index of < 2%. The NET component mostly showed diffuse positivity for serotonin and CDX2, suggesting that it consists of enterochromaffin cells. Diffuse p53 immunostaining was observed only in the high-grade adenomatous component of one case. No recurrence was observed during the follow-up period of 2-94 months. Correct distinction between the NET and poorly differentiated carcinoma components is crucial to prevent overtreatment of gastric MANETs. Considering its indolent nature, endoscopic resection is the primary recommendation for gastric MANETs as well as for pure adenomas.