A 78-year-old woman with multiple comorbidities underwent screening colonoscopy. A 15-mm homogeneous granular laterally spreading tumour (LST) was detected straddling the lower lip of the ileocecal valve. The lesion showed regular surface and vascular patterns and was classified as NICE I on NBI, consistent with sessile serrated lesion (SSL). We adopted the “novel underwater” technique: after complete gas aspiration and water filling, submucosal injection of saline plus indigo carmine (IC) was performed directly under water immersion using a standard 23-G needle. The cushion created excellent lift and vivid blue demarcation of the flat borders. En bloc underwater endoscopic mucosal resection (UEMR) was then carried out using a diathermic stiff snare. Inspection of the resection bed revealed no residual lesion, bleeding, or perforation. One prophylactic clip was applied. Histopathology confirmed a SSL without dysplasia and with negative margins.

Conventional EMR (CEMR) and UEMR show comparable technical success and adverse event rates, although recurrence is higher after CEMR. Most studies predominantly include adenomas, with SSLs representing a minority. SSLs remain challenging due to their subtle margins. While IC chromoendoscopy may assist in demarcation, its use before UEMR can impair visibility. Perforation has been reported when injection precedes water filling. Conversely, a recent Japanese study demonstrated that submucosal injection performed directly under water immersion ('novel underwater') is safe and effective.

This case study shows that the 'novel underwater' technique is safe and dramatically improves margin delineation in challenging SSLs. To our knowledge, this is the first reported use of this technique for treating colorectal lesions outside of Japan.