Invasive Colon Cancer Inadvertently Resected by Cold Snare Polypectomy.

A 74-year-old man with anemia underwent colonoscopy, which revealed a 4-mm polyp in the ascending colon. The polyp was subsequently diagnosed as an adenomatous lesion according to the narrow-band imaging (NBI) International Colorectal Endoscopic classification/Japan NBI Expert Team classification and resected via cold snare polypectomy (CSP). However, a pathological examination revealed a well-differentiated adenocarcinoma with a positive vertical margin. We performed additional endoscopic resection at the CSP scar area, revealing residual submucosal cancer with lymphatic involvement. The patient then underwent additional surgical resection. In such cases, additional endoscopic resection might be a treatment option.