Biliary tract can be anatomically divided by intrahepatic, perihilar and distal bile duct. Biliary tract is a unique organ in tumor staging scheme, because it is very long in total length and its surrounding layer is different by location. In intrahepatic portion it is surrounded by liver parenchyma, so most intrahepatic bile duct cancer should follow the staging scheme of hepatocellular carcinoma. In hilar portion, proximal half of bile ducts are covered by liver parenchyma and the remaining distal portion is surrounded by perihilar soft tissue. The last distal bile duct distal bile duct, it is surrounded by thin periductal adipose tissue and pancreas. In general, tumor staging is based on depth of tumor invasion, nodal metastasis and distant metastasis. In gastrointestinal tract malignancy, tumor staging is not difficult as its’ surrounding layer or organ is relatively uniform. In hollow viscera organ, such as stomach and intestine, basic scheme of tumor invasion can be easily evaluated by layering of mucosa, muscularis musoca, submucosa, muscle propria, and perimuscular subserosa and visceral peritoneum. However it is more difficult to evaluate depth of invasion in biliary tract cancer. First, there is no submucosal layer in bile duct and fibromuscular layer, which corresponds to muscular propria, is very thin. Second, the surrounding layer or organ is different in their anatomic location. Third, most bile duct cancer accompany desmoplastic stromal reaction that obscure the lower boundary of bile duct. Furthermore the anatomic variation of biliary tract is not uncommon. It is needed for pathologist to recognize accurate anatomy and its variation of biliary tract in each patient case for accurate tumor staging evaluation. In AJCC 8th edition, there is relatively big change in biliary tract cancer. The depth of invasion was introduced in distal bile duct cancer. It was not easy to differentiate the lower boundary of bile duct when tumor invasion occur that is why the tumor invasion depth should be used for evaluation of T stage of distal bile duct cancer. The beginning point of tumor depth should be basal line of adjacent normal bile duct, not from top of tumor. In case of intraductal papillary neoplasm of bile duct, it has been reported if tumor invasion depth start from top of intraductal papillary neoplasm it can be overestimated. In perihilar bile duct, N stage was subdivided to N1 (one to three positive nodes) and N2 (four or more positive nodes). This sub-classification may be reasonable as there are many lymph nodes around perihilar portion. It should be careful for pathologist to evaluate nodal status of biliary tract cancer. Understanding of anatomy of biliary tract and delicate gross examination may be essential in evaluation of biliary tract cancer.
Below is the summary of biliary tract cancer in AJCC 8th Edition.