Background: Endoscopic full-thickness resection (EFTR) is expected to make possible minimally invasive local resection of early gastric cancer (EGC). However, no consensus exists regarding how far an optimal safety margin should be set in determining the resection area by endoscopy. We aimed to investigate the optimal lateral margin of EGC which could be a candidate for EFTR by measuring the subepithelial extent (SE) of tumors. Methods: In 60 surgically resected submucosal EGCs 4 cm or smaller, 595 prepared slides which showed lateral tumor borders both on the epithelial surface and on the subepithelial layer were assessed. The distance between the epithelial and the subepithelial tumor edge was measured under microscopic observation, followed by analyses of the relationship between the measured SE and the histological characteristics. Results: The average and the median SE were 1.1 mm (standard deviation 1.8 mm) and 0.3 mm (range 0–12.3 mm), respectively. The 99th percentile was 8.8 mm. With regard to the histological type, the median SE was significantly greater in diffuse-type tumor than in intestinal-type tumor (0.9 mm vs 0 mm, p < 0.0001). With regard to the location of the subepithelilal tumor edge, the median SE was significantly greater in the submucosal layer than in the mucosal layer (2.6 mm vs 0.3 mm, p < 0.0001). Conclusions: In most lesions, the SE was less than 1 cm. A safety margin may be set at 1 cm in EFTR of submucosal EGC.
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