Hereditary diffuse gastric cancer with carcinogenesis from ectopic gastric mucosa in the duodenum.

A 43-year-old man with a family history of gastric cancer had undergone endoscopic submucosal dissection (ESD) for a signet-ring cell carcinoma (SRCC) lesion. Five years later, a recurrent SRCC lesion was treated with a second ESD. Two months later, multiple recurrent SRCC lesions were again detected. Immunohistochemical staining revealed reduced E-cadherin expression. Genetic testing for CDH1 identified an R732Q missense mutation. The patient subsequently underwent total gastrectomy, which identified 22 intramucosal SRCC lesions. Notably, the most distal carcinoma was located in the duodenal bulb and appeared to arise from mucosa containing gastric fundic glands, gastric foveolar epithelium, CD10-positive cells, MUC2-positive cells, and Brunner's glands, suggesting origin from ectopic gastric mucosa in the duodenum. To our knowledge, this is the first reported case of SRCC arising from ectopic gastric mucosa in patients with hereditary diffuse gastric cancer. Screening for ectopic gastric mucosa may be warranted in patients with CDH1 mutations.