INTRODUCTION: Cholangiocarcinoma (CCA) are sub-divided into intrahepatic (iCCA) or extrahepatic (eCCA). eCCA are further subdivided into perihilar (pCCA) and distal (dCCA). Current and previous versions of the WHO International Coding of Disease and Oncology classifications (ICD) have separate topography codes for iCCA and eCCA, but none for pCCA. Over recent decades, multiple studies report rising incidence rates of iCCA with declining rates of eCCA, without reference to pCCA. We hypothesised the lack of a specific code for pCCA has led to errors CCA coding, specifically with miscoding of pCCA as iCCA. METHODS: Clinical notes of cases coded as hepatobiliary carcinoma using ICD-10 criteria (C22.1/Intrahepatic Bile Duct carcinoma, C24.0/Extrahepatic Bile Duct carcinoma, C23X/Malignant Neoplasm Gall Bladder, C22.0/Malignant Neoplasm Liver Cell Carcinoma) over a 2 year period (2015-2017), were reviewed by two independent clinicians at three independent UK regional HepatoPancreatoBiliary centres. The agreed final diagnosis was compared to the originally allocated ICD-10 code. RESULTS: Of the 625 CCA cases fully reviewed, 226 were coded as C22.1/iCCA. 98 (43%) of these were true iCCA and coded correctly, while 76 cases (34%) were actually pCCA. 92% all pCCA cases were incorrectly coded as iCCA. CONCLUSION: CCA coding misclassification in UK HPB centres is common, particularly the miscoding of pCCA, which is extrahepatic and the commonest form of CCA, as iCCA. This may be contributing to apparent rising incidence rates of iCCA. Our findings confirm the need to implement distinct topographical codes for iCCA, pCCA and dCCA in future iterations of ICD
