AbstractBackground/purposeThe aim of this study was to clarify the association between the DNA damage response mediated by p53‐binding protein 1 (53BP1) in residual carcinoma in situ at ductal stumps and local recurrence in patients undergoing resection for extrahepatic cholangiocarcinoma.MethodsA retrospective analysis was conducted of 11 patients with positive ductal margins with carcinoma in situ. To evaluate the early DNA damage response, the nuclear staining pattern of 53BP1 was examined by immunofluorescence. TUNEL analysis was used to calculate the apoptotic index.ResultsOf the 11 tumor specimens of carcinoma in situ, seven showed diffuse localization of 53BP1 in nuclei (53BP1 inactivation) and four showed discrete nuclear foci of 53BP1 (53BP1 activation); the apoptotic index was significantly decreased in the seven tumor specimens with 53BP1 inactivation compared to the four with 53BP1 activation (median apoptotic index, 1 vs. 22 %;p = 0.003). The cumulative probability of local recurrence was significantly higher in patients with 53BP1 inactivation than in patients with 53BP1 activation (cumulative 5‐year local recurrence rate, 60 vs. 0 %;p = 0.019).ConclusionsClinically evident local recurrence of residual carcinoma in situ at ductal stumps is closely associated with 53BP1 inactivation and decreased apoptosis.