Screening esophagogastroduodenoscopy of a 65-year-old man revealed a 4.7-cm polypoid in the gastric high body. Clinical and laboratory findings, including serum gastrin level (460 pg/mL) and biopsy findings, were consistent with a diagnosis of type I neuroendocrine tumor (NET). Histologically, the mass consisted of dilated tortuous glands at the surface and grade 1 NET in deeper tissue. Some hyperplastic glands exhibited a transition to adenocarcinoma, which invaded the NET, simulating a "tumor in tumor" appearance. Next-generation sequencing revealed that the adenocarcinoma component harbored a TP53 mutation, whereas the NET component showed no pathogenic mutation. To our knowledge, this unusual collision of adenocarcinoma and NET within a single gastric hyperplastic polyp has not been previously described. This case suggests that large gastric hyperplastic polyps should be carefully examined because of the possibility of underlying NET and malignant transformation of surface epithelium.
Keywords: adenocarcinoma; hyperplastic polyp; neuroendocrine tumor; stomach.