We report the case of a man in his 70s with a history of diabetes mellitus, hypertension, and dyslipidemia. He underwent annual colonoscopic examination for colonic polyps, which revealed submucosal tumor-like elevated lesions, diffusely spread the sigmoid colon. The lesion had a smooth surface with central erosion. Endoscopic ultrasonography revealed a hypoechoic area in the submucosal layer. We performed endoscopic mucosal resection for pathological confirmation. Histopathological findings in a cut section revealed invaginated mucosa with inflammatory cell infiltration and mucin-containing cystic lesion in the submucosal layer, without any fibromuscular obliteration. Immunohistochemical analysis showed invaginated mucosa with CK20-immunopositivity and CK7-immunonegativity. Etiology underlying colonic mucosal invagination into the submucosal layer remains unclear. The patient is being followed up annually.
