Purpose. The aim of this study was to examine the characteristics of difficult cases and the learning curve in colorectal endoscopic submucosal dissection (ESD). Methods. We studied 518 colorectal tumors treated by ESD. Patients were divided into 2 groups such as the difficult ESD group and non-difficult ESD group in view of procedure time and procedure speed, respectively. The clinical features in each group were analyzed, and we also examined cases with severe fibrosis. Furthermore, we divided all cases into 5 periods according to experience of ESDs and investigated the rates of difficult and perforation cases. Results. In view of both procedure time and procedure speed, there were significant differences about mean tumor size, rates of severe fibrosis and perforation, and en bloc resection rate between the two groups. Severe fibrosis was detected in protruding tumors >40？mm in diameter. With respect to the learning curve, the rate of difficult and perforation cases decreased significantly in the late periods compared to the first period. Conclusions. Large tumor size, high rates of severe fibrosis and perforation, and low rate of en bloc resection are related with difficult ESD cases. The increasing of experiences can decrease the rate of difficult cases and perforation. 1. Introduction In Japan and some other Western and Asian countries, ESD is reported to be an efficient treatment with a high rate of en bloc resection for large colorectal tumors, and ESD is less invasive than laparoscopic colectomy (LAC) [1]. ESD should be performed for tumors that are diagnosed as intramucosal cancer and shallowly invaded submucosal cancer [2, 3]. The number of colorectal ESD has increased gradually with the development of safer strategies and improvements of suitable ESD devices. However, the control of endoscopes and ESD knives are hindered in some colorectal ESD cases because the colon is winding and has many folds. Additionally, restlessness resulting from abdominal fullness and pain is related to prolonged procedure times [4, 5]. It is therefore important to predict difficult cases to prevent complications, including perforation [5, 6]. In this study, we investigated difficult ESD cases with long procedure times or slow procedure speed and examined learning curve of ESD. 2. Patients and Methods A total of 518 tumors in 418 patients who underwent ESD at the Kyoto Prefectural University of Medicine or Nara City Hospital from 2006 to 2013 were analyzed. We examined clinical outcomes for all 518 tumors and divided the tumors into 2 groups such as the difficult
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