Introduction: Underwater endoscopic mucosal resection (UEMR) does not involve submucosal injection. Its ability to achieve sufficient resection depth comparable with that of conventional endoscopic mucosal resection (CEMR) remains unclear. This randomized controlled trial aimed to compare UEMR and CEMR, with a specific focus on resection depth.
Methods: In this noninferiority randomized controlled trial at 4 Japanese institutions, patients with colorectal tumors measuring 10-20 mm were randomly assigned to the UEMR or CEMR group. The primary outcome was resection depth analyzed per protocol, with a noninferiority margin of 400 μm.
Results: In total, 217 patients were randomly assigned to the UEMR (n = 109) or CEMR (n = 108) group. The mean resection depths were 1,688.9 ± 1,510.0 μm for UEMR and 1,432.3 ± 1,294.2 μm for CEMR, with a mean difference of 256.6 μm (95% confidence interval: -119.9-633.1 μm; P = 0.18). Subgroup analyses revealed that UEMR achieved significantly greater resection depths in the cecum and ascending colon (1,822.4 ± 1,469.7 μm vs 1,096.5 ± 1,007.6 μm; P = 0.01) and in lesions with superficial morphology (1,238.7 ± 862.2 μm vs 731.6 ± 376.1 μm; P < 0.01) than CEMR. Nonexpert operators also achieved deeper resections with UEMR than with CEMR (1,786.6 ± 1,471.6 μm vs 1,192.4 ± 1,074.7 μm; P = 0.07).
Discussion: UEMR achieved a resection depth comparable with that of CEMR. Given its consistent negative horizontal margins, UEMR may become the gold standard for resecting intermediate-sized colorectal tumors.
Keywords: colorectal tumor; conventional endoscopic mucosal resection; noninferiority; resection depth; underwater endoscopic mucosal resection.
Copyright © 2026 The Author(s). Published by Wolters Kluwer Health, Inc. on behalf of The American College of Gastroenterology.