Background and study aim Additional surgery is recommended if an endoscopically resected T1 colorectal cancer (CRC) specimen shows a positive resection margin. We aimed to investigate the significance of a positive resection margin in endoscopically resected T1 CRC. 

Patients and methods We enrolled 265 patients with T1 CRC who underwent endoscopic resection between January 2001 and December 2016. The inclusion criteria were: 1) complete resection by endoscopy, and 2) pathology of a positive margin. Among the 265 patients, 213 underwent additional surgery and 52 did not. In the additional surgery group, various clinicopathological factors were evaluated with respect to the presence or absence of residual tumor. The follow-up results were assessed in the group that did not undergo additional surgery. 

Results In the 213 patients who underwent additional surgery, residual tumor was detected in 13 patients (6.1 %), and none of the clinicopathological factors was significantly associated with the presence of residual tumor. Among the 52 patients who did not undergo additional surgery, recurrence was detected in 4 (7.7 %), and all 4 underwent salvage surgery. Among these four patients, three had no risk factors for lymph node metastasis and recurrence was at the previous resection site; pathology was high grade dysplasia, rpT3N0M0, and rpT1N0M0, respectively. 

Conclusions A positive resection margin in endoscopically resected T1 CRC is related to a relatively low incidence of residual tumor (6.1 %). Although current guidelines recommend additional surgery for such cases, surveillance and timely salvage surgery could be another option in selected cases.