A 59-year-old woman underwent esophagogastroduodenoscopy as a health check-up, which revealed a submucosal tumor-like lesion with a central recess, measuring 20 mm in diameter, on the opposite side of the major duodenal papilla. A biopsy specimen revealed an atypical gland. Although endoscopic ultrasound showed that the tumor may be submucosal invasive cancer, we performed endoscopic submucosal dissection (ESD) due to her lung cancer. Histopathological examination of the tumor showed intramucosal adenocarcinoma (pap>tub1) without vascular and lymphatic invasion. Immunostaining of the tumor revealed positivity for MUC5AC and MUC6, and negativity for CD10 and MUC2, indicating the gastric phenotype. No recurrent cancer has been observed one year after ESD. Endoscopic therapy for early duodenal cancer of the gastric phenotype might lead to the improved prognosis of the patient.
