Type 1 and 2 subclassification may be a promising approach to evaluate intraductal papillary neoplasm and intraductal oncocytic neoplasm of the bile duct.

In the pancreatobiliary system, precursor lesions are traditionally classified as low-grade dysplasia (LGD) or high-grade dysplasia (HGD), a scheme widely applied to pancreatic precursors. However, applying this system to biliary precursors, including intraductal papillary neoplasm of the bile duct (IPNB) and intraductal oncocytic papillary neoplasm of the bile duct (bIOPN), presents several challenges: (i) virtually all IPNBs and bIOPNs contain variable foci of HGD; (ii) lesions composed entirely of LGD and associated with a favorable clinical course may not exist; and (iii) virtually all IPNBs and bIOPNs are managed clinically as malignant tumors at diagnosis. Accordingly, a type 1/2 subclassification was proposed on the basis that HGD can be divided into traditional HGD and HGD with irregular and complicated lesions (ICLs), irrespective of LGD. Type 2 IPNB/bIOPN exhibits traditional HGD with ICLs, whereas type 1 exhibits traditional HGD without ICLs. Type 1 IPNB/bIOPN shares many features with prototypic pancreatic and ampullary intraductal tumoral precursors, whereas type 2 differs to varying degrees from these prototypes. Subsequent studies have demonstrated the superiority of this subclassification over traditional two-tiered grading, with moderate interobserver agreement, and have identified additional clinicopathological features of IPNB/bIOPN. Importantly, type 2 is associated with less favorable postoperative overall survival than type 1. The type 1/2 subclassification may therefore provide a useful framework for clarifying the clinicopathological and molecular/genetic features of IPNB/bIOPN.