A Step-Up Underwater Endoscopic Mucosal Resection Strategy with Rescue Underwater Injection Endoscopic Mucosal Resection for Non-Pedunculated Colorectal Lesions.

Objectives: Underwater endoscopic mucosal resection (UEMR) is an established treatment for non-pedunculated colorectal lesions, but securing an adequate resection margin can be difficult in selected cases due to poor visualization or snare instability. To address these limitations, we used a step-up strategy with UEMR first and selective conversion to underwater injection EMR (UIEMR) when necessary. This retrospective study evaluated the technical outcomes and safety of this step-up strategy for non-pedunculated lesions measuring 10-20 mm. Methods: This single-center retrospective study included 56 consecutive 10-20 mm non-pedunculated colorectal lesions treated with UEMR as the first-line approach between September 2024 and September 2025. When predefined intra-procedural criteria for UEMR difficulty were met, the procedure was converted to UIEMR. The primary endpoints were technical success (en bloc and R0 resection rates) and procedure-related adverse events of the step-up strategy. Results: Resection was completed using UEMR alone in 31 lesions, whereas 25 lesions required conversion to UIEMR. En bloc resection was achieved in all lesions, and histologically negative margins were reported in all cases. No delayed bleeding or perforation was observed. Conclusion: For 10-20 mm non-pedunculated colorectal lesions, a step-up strategy with UEMR first and selective conversion to UIEMR was feasible and safe and may be a practical treatment option.Clinical trial registration: N/A.