Pancreatic Invasion in Distal Cholangiocarcinoma Is a Poor Prognostic Factor: A New Approach Integrating Tumor Depth of Invasion

Background The 8th edition of the American Joint Committee on Cancer (AJCC) staging system for distal cholangiocarcinoma (dCCA) uses depth of invasion (DOI) for T classification; however, it does not consider invasion into adjacent organs such as the pancreas. We investigated the prognostic significance of DOI and pancreatic invasion in patients with dCCA. Methods We retrospectively analyzed 130 patients with dCCA who underwent pancreatoduodenectomy between 2003 and 2016. The optimal DOI cutoff for predicting overall survival (OS) was determined using time‐dependent receiver operating characteristic analysis. Associations between clinicopathological factors, pancreatic invasion, and prognosis were evaluated using Kaplan–Meier survival analysis and Cox proportional hazards models. Results The optimal DOI cutoff for OS was 4 mm. Patients with DOI > 4 mm had significantly poorer survival than those with DOI ≤ 4 mm ( p < 0.0001). Pancreatic invasion was associated with worse prognosis ( p < 0.001) and was more prevalent in tumors with greater DOI. Among patients with DOI > 4 mm, pancreatic invasion significantly worsened OS ( p < 0.05), whereas no prognostic impact was observed in patients with DOI ≤ 4 mm. Multivariate analysis identified the combination of DOI > 4 mm and pancreatic invasion, lymph node metastasis, histological type, and positive resection margin as independent adverse prognostic factors. A novel classification incorporating DOI, pancreatic invasion, and lymph node status provided improved prognostic stratification. Conclusion The combination of DOI > 4 mm and pancreatic invasion is an independent predictor of poor prognosis in dCCA. Incorporating pancreatic invasion into DOI‐based classification may improve prognostic assessment beyond the current AJCC staging system.