Aims: The aims of the present study were to evaluate the feasibility of endoscopic submucosal dissection (ESD) as curative treatment for node-negative submucosal invasive early gastric cancer (EGC) and to consider further expansion of the curability criteria for submucosal invasive EGC. Methods: A total of 977 EGC in 855 patients treated by ESD were enrolled. They were divided into intramucosal cancer (M); minimally submucosal invasive cancer (<500 μm from the muscularis mucosa) (SM1); and deeper submucosal invasive cancer (>500 μm from the muscularis mucosa) (SM2). The technical feasibility of ESD for SM1 and M were compared, and the clinical prognosis of SM1 was evaluated. Furthermore, the volume of carcinoma invading to the submucosal layer, which we called the SM volume index, was calculated virtually to analyze its correlation with lymphatic-vascular invasion. Results: There were no statistical differences in technical outcomes and complications between M and SM1. Curative resection rates were significantly better in M than in SM1 (M, 92.6%; SM1, 63.8%). No local recurrences and distant metastases were found in 48 SM1 patients declared to have undergone curative resections. Most cases (72.0%) with successful ESD but non-curative resection exceeded 30 mm in maximum size, and no local recurrences and metastases were found in these patients. The SM volume index of these cases was comparatively small. Conclusion: The technical and theoretical validity of ESD for SM1 was validated. The possibility of further expansion of the curability criteria for submucosal invasive cancers was suggested by the evaluation of the SM volume index.
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