Gastric adenocarcinoma with submucosal invasion occurs in a sporadic fundic gland polyp of a Helicobacter pylori-uninfected patient on proton pump inhibitor.

Gastric fundic gland polyps (FGPs) are common benign lesions, typically found in Helicobacter pylori (HP)-uninfected stomachs. While syndromic FGPs resulting from genetic diseases may exhibit dysplasia, sporadic FGPs rarely develop into adenocarcinomas. Here, we present the first case of invasive gastric adenocarcinoma that occurred in a sporadic FGP in an HP-uninfected patient. A 77-year-old man on proton pump inhibitor (PPI) therapy for 6 years developed an enlarged reddish FGP. The absence of atrophic changes in the entire gastric mucosa was confirmed endoscopically and multiple HP tests were negative. After PPI discontinuation, the lesion initially reduced in size from 25 to 15 mm in 1 month; however, malignancy was suspected because of rapid enlargement and persistent discoloration. En bloc resection was performed by endoscopic mucosal resection. Histological examination confirmed FGP and submucosal adenocarcinoma with a depth of approximately 700 μm, negative for lymphovascular invasion, and negative resection margins. A subsequent distal gastrectomy confirmed the absence of residual disease or lymph node metastases. The unusual endoscopic findings of rapid enlargement and reddish tone pushed us to perform endoscopic treatment, which resulted in a correct diagnosis and appropriate treatment.