Small Duodenal Adenocarcinoma with Peritoneal Dissemination Mimicking Duodenal Brunner's Gland Hyperplasia.

Introduction: Gastric-phenotype non-ampullary duodenal carcinomas are rare but potentially aggressive tumors.

Case presentation: A 52-year-old woman was referred for a duodenal polyp. Endoscopy revealed multiple duodenal bulb polyps suggestive of Brunner's gland hyperplasia. Histopathology confirmed well-differentiated tubular adenocarcinoma with submucosal invasion and a positive margin. The patient underwent pancreaticoduodenectomy, and residual subserosal carcinoma was detected. Despite adjuvant chemotherapy, liver metastases and peritoneal dissemination developed 7 months postoperatively. The patient died 24 months after surgery.

Conclusion: This case highlights the aggressive nature of gastric-phenotype duodenal adenocarcinoma and the need for early radical surgical intervention when vertical margins are positive after endoscopic resection.