Impact of changes in the topographic classification of Klatskin tumor on incidence of intra‐ and extrahepatic bile duct cancer: A population‐based national cancer registry study

Background Misclassification of Klatskin tumor as intrahepatic rather than extrahepatic bile duct cancer (eBDC) may overestimate the incidence of intrahepatic BDC (iBDC). This study aimed to investigate the impact of misclassified Klatskin tumor on iBDC and eBDC incidences in Korea, where BDC is highly endemic. Methods The data of incidence, topography and morphology code of BDC from 1999 to 2017 were obtained from the Korea Central Cancer Registry, which covers the entire 51.6 million Korean population. For misclassification analysis, all Klatskin tumors were reclassified as eBDC. Results Klatskin tumors accounted for 13.5% of all 81 414 BDC cases. In the registry, an average of 59.7% of Klatskin tumors were classified as iBDC, gradually decreasing from 95.5% to 16.9%. Misclassification led to a 17.3% overestimation of iBDC cases and a 15.0% underestimation of eBDC cases on average. After reclassification, age standardized rate (ASR) of incidence per 100 000 population decreased in iBDC from 3.4 to 2.9 and increased in eBDC from 2.8 to 3.2. Average annual percentage change of iBDC and eBDC incidences were 2.0% and 1.2%, respectively, but ASR of iBDC significantly decreased since 2012 (P