Background:Diagnosing the invasion depth of ulcerated early gastric cancer is usually inaccurate, especially for endoscopists in primary care clinics who are often not experts in this area. In reality, many patients with open ulcers who can be treated with endoscopic submucosal dissection (ESD) are referred for surgery.Materials and methods:Twelve patients with ulcerated early gastric cancer who were treated with proton pump inhibitors, including vonoprazan, and underwent ESD were included in the study. Conventional endoscopic and narrow-band images were evaluated by five board-certified endoscopists (two physicians: A, B, and three gastrointestinal surgeons: C, D, and E). They assessed the invasion depth, and the results were compared with the pathologic diagnosis.Results:The accuracy of the invasion depth diagnosis was 38.3%. According to the pretreatment diagnosis of invasion depth, gastrectomy was recommended for 41.7% (5/12) of the cases. However, histological examination revealed that additional gastrectomy was required in only one case (8.3%). Thus, in four out of five patients unnecessary gastrectomy could be avoided. Post-ESD mild melena occurred in only one case, and there was no case of perforation.Conclusion:Antiacid treatment contributed to avoid unnecessary gastrectomy in four out of five patients for whom gastrectomy was indicated based on an inaccurate pretreatment diagnosis of the invasion depth.