Pathological diagnosis of flat epithelial lesions of the biliary tract with emphasis on biliary intraepithelial neoplasia.

Flat epithelial lesions of the biliary tract cannot be detected by the image analysis, and the diagnosis entirely depends on pathological examination. The biliary tract is often affected by inflammatory conditions, and the resultant changes of the biliary epithelium make it difficult to differentiate them from neoplasia. Thus, the pathological diagnosis of biliary flat epithelial lesions can be challenging. In the biliary tract, there are several forms of intraepithelial neoplasia of the flat type, and biliary intraepithelial neoplasia (BilIN) is known as one of such lesions that represent the multistep cholangiocarcinogenesis. In this article, the diagnostic criteria and the differential diagnosis of biliary flat epithelial lesions, particularly focusing on BilIN, were presented and discussed to provide help to advance clinical and research applications of the BilIN system.