Autoimmune pancreatitis and biliary intraepithelial neoplasia of the common bile duct: A case with diagnostically challenging but pathogenetically significant association

Autoimmune pancreatitis (AIP) (also called IgG4‐related sclerosing pancreatitis (IgG4‐SP)) and IgG4‐related sclerosing cholangitis (IgG4‐SC) are frequently associated with each other. It is generally believed that association of these diseases with pancreatobiliary malignancy is, however, rare. Here, we report on the case of a patient with AIP whose biliary cytology revealed severely atypical cells. Surgically resected specimens from this patient showed typical AIP with IgG4‐SC, as well as a mildly elevated lesion in the common bile duct with varying degrees of cellular atypia. In addition, the atypical cells tested positive for the mucin‐core protein, MUC5AC and p53 overexpression. These findings led us to diagnose the common bile duct lesion as biliary intraepithelial neoplasia (BilIN, mainly BilIN‐1/2). Recently, associations between K‐ras mutations and pancreatobiliary carcinoma have been reported in patients with AIP. This case, therefore, provides important new insight into the potential association of AIP and/or IgG4‐SC with malignancy (or precursor lesions) of the pancreatobiliary system.