Inverted Heterotopic Gastric Mucosa of the Duodenum Presenting as a Subepithelial Lesion

A 53-year-old man underwent screening esophagogastroduodenoscopy, which revealed a 20-mm subepithelial lesion (SEL) in the second portion of the duodenum. The patient was asymptomatic and had no relevant medical history. He was referred to our hospital for further evaluation. White-light endoscopy showed a sessile lesion covered with smooth non-neoplastic duodenal mucosa, with multiple depressed orifices over its surface (Figure 1a). On magnifying narrowband imaging (NBI), the surface of the lesion exhibited a villous structure consistent with normal duodenal mucosa, whereas glandular structures resembling gastric mucosa were observed inside the orifices (Figure 1b). Furthermore, endoscopic ultrasonography (EUS) demonstrated an anechoic region surrounded by multiple septations confined to the submucosal layer (Figure 1c). Biopsy specimens from the orifice revealed gastric fundic glands predominantly composed of chief cells, with surface epithelium showing foveolar differentiation. These findings could not completely exclude a gastric-type duodenal neoplasm. Because EUS findings showed that the lesion was confined to the submucosa, we performed en bloc underwater endoscopic mucosal resection for both diagnostic and therapeutic purposes. Histology revealed proliferation of gastric foveolar epithelium and fundic glands with an inverted submucosal growth pattern (Figure 2a). The anechoic area on EUS corresponded histologically to dilated gastric glands. Immunohistochemically, the gastric foveolar epithelium was diffusely positive for MUC5AC, whereas the fundic glands were diffusely positive for MUC6 and partially positive for pepsinogen I (Figure 2b–d). In the absence of cytologic atypia, a diagnosis of heterotopic gastric mucosa was made. Desmin staining clearly demonstrated inverted glandular proliferation disrupting the muscularis mucosae and extending into the submucosa (Figure 2e). Reports of duodenal heterotopic gastric mucosa presenting as a large SEL-like lesion are rare [1, 2]. As heterotopic gastric mucosa has been proposed as a potential precursor lesion or origin of gastric-type neoplasms [3], endoscopic resection was considered an appropriate management in this case. EUS was useful for assessing subepithelial involvement and selecting an appropriate treatment. We would like to thank Editage (www.editage.jp) for English language editing. The authors received no specific funding for this work. This report adhered to the Declaration of Helsinki. According to the policy of the Ethics Committee of Osaka International Cancer Institute, case reports without research characteristics (including non-research case reports such as the present one) are exempt from ethics committee approval. Informed consent was obtained from the patient. Ryu Ishihara has received honoraria from Olympus Corporation, FUJIFILM Medical Co. Ltd., Daiichi Sankyo Co. Ltd., Miyarisan Pharmaceutical Co. Ltd., AI Medical Service Inc., AstraZeneca, MSD, Ono Pharmaceutical Co. Ltd., and KYORIN Pharmaceutical Co. Ltd. The other authors declare no conflicts of interest. The data that support the findings of this study are not openly available due to reasons of sensitivity and are available from the first or corresponding author upon reasonable request.