Spectrum of Biliary and Nonbiliary Neoplasms Growing and Spreading Within the Lumen of the Bile Ducts

In the hepatobiliary system, the majority of neoplasms grow within the hepatic parenchyma; however, some arise, grow, and/or spread within the lumen of the intrahepatic large bile ducts and the perihilar/distal bile ducts (collectively referred to as large bile ducts), representing specialized ductal organs associated with unique peribiliary glands and being distinct from the intrahepatic small bile ducts and bile ductules embedded within the hepatic parenchyma. Precursors of cholangiocarcinoma (CCA) arising within the lumen of large bile ducts have recently been proposed. Neoplasms growing and spreading within the lumen of large bile ducts have been categorized into four groups and are discussed here in light of updated pathological findings. (i) Precursor(s) of CCA arising in the large bile ducts (large-duct-type intrahepatic CCA and perihilar/distal CCA): These precursors include high-grade biliary intraepithelial neoplasia (BilIN), intraductal papillary neoplasm of the bile duct (IPNB), and intraductal oncocytic papillary neoplasm (IOPN). High-grade BilIN presents as a flat, microscopic lesion with dysplastic cytoarchitectural alterations and grows along the luminal surface of large bile ducts, whereas the latter two present as grossly visible polypoid or tumorous lesions composed of papillary, villous, or tubular proliferation of neoplastic epithelium with delicate fibrovascular cores. These lesions may eventually progress to invasive CCA. Intraductal tubulopapillary neoplasm of the bile duct (ITPN), previously categorized as another precursor of CCA arising in large bile ducts, appears to represent a heterogeneous group of neoplasms with respect to progression and presumed cell of origin. Some ITPNs are frequently associated with nodular invasive carcinoma resembling small-duct-type intrahepatic CCA (SD-iCCA) and share genetic alterations with SD-iCCA; such cases may arise in association with small bile ducts or bile ductules. In contrast, other ITPNs exhibit cystic changes with tubulopapillary features and may arise in association with peribiliary glands or cysts. (ii) Secondary growth and spread of biliary neoplasms: This category comprises several patterns. First, intraepithelial neoplastic spread directly and continuously from the primary neoplastic lesion is observed in almost all cases of high-grade BilIN, IPNB, and IOPN; it spreads laterally along the luminal surface of the proximal and distal bile ducts and extends vertically into the adjacent peribiliary glands. Intraluminal cast-like spread in the bile ducts adjacent to the primary neoplastic lesion also occurs in some precursor lesions, particularly in ITPN. Implantation of a biliary neoplasm from one part of the biliary tract to another results in discontinuous, multifocal biliary neoplasms, particularly in IPNB, and occurs mainly in the distal bile ducts relative to the main tumor. Multicentric tumorigenesis may contribute to the multifocal development of precursors and CCA in the bile ducts. The accumulation of additional genetic alterations, beyond the common mutations detected in primary tumors, may contribute to metachronous recurrence of CCA after curative resection of the primary biliary tumor. Cancerization of the duct (COD) by CCA may also contribute to secondary growth and spread within the bile duct lumen. Specifically, flat-type cancerization of pre-existing non-neoplastic bile ducts, resembling high-grade BilIN, occurs in approximately one-third of hilar CCA cases. Intraductal polypoid, cast-like cancerization within the lumen of adjacent bile ducts, resembling polypoid precursors of CCA, can also occur in approximately one-tenth of SD-iCCA. (iii) Prominent intraductal polypoid growth of invasive CCA: Invasive CCA rarely presents with predominant intraductal polypoid carcinoma that is continuous with periductal infiltrating CCA; this pattern can be referred to as polypoid invasive CCA. (iv) Nonbiliary neoplasms presenting bile duct tumor thrombus (BDTT): BDTT associated with hepatocellular carcinoma and with extrahepatic malignancies extending into the bile duct lumen can mimic the intraluminal growth and spread patterns of the above-mentioned biliary neoplasms. In conclusion, intraluminally growing biliary neoplasms in the large bile ducts comprise a heterogeneous group that can be reasonably classified into four categories. This categorization may facilitate understanding of these intrabiliary growing neoplasms.