Background : Composite glandular/exocrine-endocrine neoplasms of the gastrointestinal tract are a special tumor type. There are only three reports in English on secondary gastric neuroendocrine tumor (G-NET) arising from gastric adenoma. Here, we describe a rare case of a G-NET arising from a gastric adenoma. Case presentation: A 69-year-old man underwent esophagogastroduodenoscopy as part of a general health check-up. An endoscopic examination revealed a 5-mm elevated tumor on the fornix of the stomach. Further, atrophic changes in the gastric mucosa were not noted. The tumor was diagnosed as a gastric adenoma; thus, endoscopic submucosal dissection (ESD) was performed. Histologically, the tumor was composed of two components, an epithelial component, which was diagnosed as an adenoma, and a depth component, which was diagnosed as a neuroendocrine tumor. The neuroendocrine tumor was diagnosed as a Grade 2 G-NET arising from the gastric adenoma. Histologically, the tumor invaded the submucosal layer (>1000 μm), invading the lymph vessels, additional gastrectomy was performed. Pathologist revealed no remaining tumor or lymph node metastases in the resected specimen. Conclusions : We report this extremely rare case of a G-NET (Grade 2), initially diagnosed as a gastric adenoma, that is considered to have originated from the gastric adenoma. Pathologists, endoscopists and surgeons should be aware of the occurrence and association of NETs with adenomas in the stomach because small submucosal NETs of the stomach have relatively high metastatic rates.

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