Colorectal endoscopic submucosal dissection (ESD) is challenging despite its usefulness. Underwater ESD (UESD) provides better traction and a clearer view of the submucosal layer than conventional ESD (CESD). This study compared the efficiency of UESD and CESD for large (20-50 mm) laterally spreading tumors (LSTs).

Preplanned sample size was calculated from our previous experience. As a result, 28 patients were required for the UESD group and CESD group each. The primary outcome was total procedure time; the secondary outcome was dissection speed.

Fifty-six patients were enrolled, and a total of 28 patients were assigned to each group. The mean LST size was 31.6 mm and 31.3 mm in the UESD and CESD groups, respectively. Fibrosis was observed in 67.9% and 60.7% of patients in the UESD and CESD groups. Total procedure time (mean ± standard deviation) for the UESD group was significantly shorter than that for the CESD group (49.5 ± 20.3 minutes vs 75.7 ± 36.1 minutes; mean difference, –26.2 minutes; 95% confidence interval, –42.0 to –10.5 minutes). Dissection speed of the UESD group was significantly faster than that of the CESD group (21.9 ± 6.9 mm²/min vs 15.2 ± 7.3 mm²/min; mean difference, 6.7 mm²/min; 95% confidence interval, 2.8 to 10.4 mm²/min). There was no difference between groups in the R0 resection rate or en bloc resection rate. No perforations were observed in either group.

UESD was superior to CESD in total procedure time and dissection speed. UESD can be recommended as the preferred method for the resection of large LSTs.