Hidden in Plain Sight: Gastric Adenocarcinoma in Endoscopically Normal Mucosa in a CDH1 Mutation Carrier

Hereditary diffuse gastric cancer associated with cadherin 1 (CDH1) mutations poses diagnostic challenges because of its subepithelial and multifocal nature. We report a 71-year-old woman with a germline CDH1 mutation and complex breast cancer history who chose endoscopic surveillance over prophylactic gastrectomy. Despite a grossly normal esophagogastroduodenoscopy, random biopsies revealed signet ring cell adenocarcinoma. Staging imaging was negative. She underwent total gastrectomy, and final pathology showed multifocal intramucosal carcinoma (pT1a) without nodal involvement. This case highlights the limitations of endoscopic surveillance and underscores the importance of risk-reducing surgery in CDH1 mutation carriers.