Recently, much progress has been made in the concept and classification of benign tumors and precursors in the pancreato-biliary system [[1], [2], [3], [4], [5]]. Pancreatic precursors, such as intraductal papillary mucinous neoplasm (IPMN), intraductal onocytic papillary neoplasm (IOPN), and intraductal tubulopapillary neoplasm (ITPN) have been characterized clinicopathologically [[1], [2], [3],6,7]. In the bile duct, tumoral (grossly visible) intraductal pre-invasive neoplasms (TIDN) including intraductal papillary neoplasm of bile duct (IPNB), intraductal oncocytic papillary neoplasm (IOPN) and intraductal tubulopapillary neoplasm (ITPN) are also reported [1,2,7,8]. However, there have been several controversies regarding the pathologic classification and criteria of these biliary pre-invasive neoplasms [4,5,[7], [8], [9], [10], [11]].

These pancreato-biliary intraductal preinvasive neoplasms are recently recommended to be graded as low-grade or high-grade dysplasia (LGD/HGD) based on their cytoarchitectural alterations [3,9]. However, whether the same criteria of grading of pancreatic precursors are applicable to biliary counterparts remains unclear. Almost all cases of IPNB, IOPN, and ITPN may be malignant or potentially malignant neoplasm [5,6,9,10,12], and surgical resection of these neoplasms is recommended at the time of the detection or diagnosis [5,7,13]. However, in IPMN, some cases with HGD associated with invasive carcinoma require surgical resection, while observation is recommended in others [1,3,9]. Therefore, a modified criteria different from their pancreatic counterparts is needed for the pathologic grading of TIDN.

Recently, the Japan-Korea study group proposed types 1 and 2 subclassification to characterize IPNB, based on several clinicopathological features including cytoarchitectural alterations [13,14]. However, there have been no studies on the subclassification system itself.

In this study, 10 biliary pathologists classified 227 cases of TIDN or related lesions in the bile duct into IPNB, IOPN, and ITPN and “others”. These cases of TIDN were then graded according to the traditional two-tiered grading system [3,9] and the type 1 and 2 subclassification [13,14]. Then, the interobserver agreement of the classification of TIDN, the two-tiered grading, and the type 1/2 subclassification was assessed.