Gastric adenocarcinoma of fundic gland mucosa arising in heterotopic gastric mucosa of the duodenum: A case report.

Background: Heterotopic gastric mucosa (HGM) is the most common type of epithelial heterotopia, which can occur in any part of the gastrointestinal tract. The duodenum is one of the common sites of HGM. HGM is usually considered benign, and malignant transformation of HGM in the duodenum is extremely rare.

Case summary: We reported a middle-aged man admitted to the hospital due to abdominal pain, whose gastroduodenoscopy revealed a polypoid uplift in the duodenum. The attending physician directly removed the lesion using a snare, histopathological analysis demonstrated multidirectional cellular differentiation on the basis of duodenal HGM. The hematoxylin and eosin stains of the duodenal lesion demonstrated HGM, the superficial mucosa exhibited gastric foveolar-type epithelium, numerous mucous glands were identified beneath the foveolar region. Additionally, focal areas displayed several signet-ring cell changes. Gastric foveolar-type epithelium was diffusely positivity for MUC5AC and Ki67. Numerous mucous gland was positivity for MUC6, partially positive for MUC2, pepsinogen I and H⁺/K⁺ ATPase. It was eventually diagnosed gastric adenocarcinoma of fundic gland mucosa on the basis of duodenal HGM.

Conclusion: We reported the first case of gastric adenocarcinoma of fundic gland mucosa arising in the duodenum on the basis of HGM. Although HGM is mostly benign, there is also a risk of carcinogenesis.