Reappraisal of pathological features of intraductal papillary neoplasm of bile duct with respect to the type 1 and 2 subclassifications.

The pathological spectrum of intraductal papillary neoplasm (IPNB) remains to be clarified. A total of 186 IPNBs were pathologically examined using the type 1 and 2 subclassifications proposed by Japanese and Korean biliary pathologists incorporating a two-tiered grading system (low-grade and high-grade dysplasia), with reference to four subtypes (intestinal (i), gastric (g), pancreatobiliary (pb) and concocytic (o) subtype). IPNBs were classifiable into type 1 composed of low-grade dysplasia and 'high-grade dysplasia with regular structures' (69 IPNBs), and type 2 of 'high grade dysplasia with irregular structures and complicated lesions' (117 IPNBs). Type 1 was more common in the intrahepatic bile duct (78%), while type 2 was frequently located in the extrahepatic bile duct (58%). Mucin hypersecretion was more common in type 1 (61%) than in type 2 (37%). IPNBs were classifiable into the four subtypes: 86 iPNBs, 40 gIPNBs, 31 pbIPNBs, and 29 oIPNBs. The four subtypes were histologically evaluable with reference to the type 1 and type 2 subclassifications. iIPNB and pbIPNBs were frequently classified as type 2, while types 1 and 2 were observed at similar rates in gIPNB and oIPNB. Stromal invasion was almost absent in type 1, irrespective of subtype, but was found in 66 of 117 type 2 IPNBs (p<0.01), and post-operative outcome was favorable in IPNBs without invasion compared with IPNBs with invasion (p<0.05). The type 1 and 2 subclassification with reference to the four subtypes, may provide useful information for understanding IPNB.