Validation of the T category for distal cholangiocarcinoma: Measuring the depth of invasion is complex but correlates with survival

According to the current 8th edition of the American Joint Committee of Cancer (AJCC), the T category of distal cholangiocarcinomas is classified based on the depth of invasion (DOI) (T1, < 5 mm; T2, between 5 and 12 mm; T3, > 12 mm). In consideration of the discrepancies between previous studies about the prognostic significance, we aimed to validate the current AJCC T staging system of distal cholangiocarcinomas. DOI was measured using three different methods: DOI1, DOI2, and DOI3. DOI1 was defined and stratified according to the AJCC 8th edition. DOI2 was measured as the distance from an imaginary curved line approximated along the distorted mucosal surface to the deepest invasive tumor cells. DOI3 was defined as the total tumor thickness. DOI2 and DOI3 were also divided into three categories using the same cut-off points as in the AJCC 8th edition. We compared these three DOI methods to the AJCC 7th edition as well. In contrast with the AJCC 7th edition, all three groups showed a correlation with patients' overall survival. Above all, the DOI2 group demonstrated the best significance in multivariate analysis. However, when the C indices were compared between these groups, differential significance proved to be negligible (DOI1 vs DOI2, p = 0.915; DOI2 vs DOI3, p = 0.057). Therefore, the measurement of DOI does not need to be rigorously and stringently performed. In conclusion, we showed that the current T classification system better correlates with the overall survival of patients with distal cholangiocarcinomas than the previous system.