Before ESD, the lesion will be observed either by pCLE or by CE to determine proximal and distal margin of the lesion. In the pCLE group, after injection of intravenous fluorescein, the lesion will be observed using a flexible confocal miniprobe. In the CE group, the lesion will be observed conventionally under white light after spraying indigocarmine directly via the forceps channel. Marking dots will be placed on both margins using needle knife. Then several dots will be marked on the surrounding normal mucosa 5 mm from the tumor with Argon plasma coagulation. After injection of saline solution with epinephrine into the submucosa, an initial cut will be made with needle knife outside the line of surrounding marks. An insulation-tipped (IT) knife will be inserted into initial cut, and electrosurgical current will be applied to complete the incision around the tumor. After the circumferential cut, the submucosa will be dissected with the IT knife.

Complete resection is defined as en bloc resection with negative horizontal, vertical margin and distance from marking dot to histologic margin will be measured.