Endoscopic submucosal dissection for a colorectal tumor with coexisting muscle-retracting sign and underlying diverticulum: the first reported case.

Endoscopic submucosal dissection (ESD) for colorectal lesions becomes particularly challenging in the presence of the muscle-retracting sign (MRS), and the risk of perforation is further increased when a diverticulum underlies the lesion. We report a case of ESD for a 25-mm Paris type 0-Is tumor in the sigmoid colon surrounded by multiple diverticula. During ESD, a broad MRS obscured the appropriate submucosal plane. Careful dissection revealed a diverticulum directly beneath the lesion, indicating a focal defect of the muscularis propria. Subsequently, an intraoperative perforation occurred but was immediately detected and successfully closed using endoscopic clips, allowing completion of en bloc resection. Postoperative computed tomography demonstrated minimal free air, and the patient recovered uneventfully with conservative management. This case highlights that even in an extremely high-risk situation involving both an MRS and an underlying diverticulum, careful dissection and prompt endoscopic closure may avoid surgery and achieve curative resection.