PanIN and IPMN are intraepithelial neoplasias of the pancreatic duct. PanIN was proposed in 1994 as a precursor lesion of invasive ductal adenocarcinoma of the pancreas , while IPMN has been recognized since 1982  as a distinct group of lesions with an indolent prognosis. So far, these two categories of disease have tended to be morphologically confused as intraepithelial lesions. In 2003, a meeting of international experts on the precursor lesions of pancreatic cancer was held, and the basic definitions of both IPMN and PanIN were again confirmed. The differential diagnosis between IPMN and PanIN was discussed as follows: IPMN arises in the main pancreatic duct or its major branches and usually produces lesions greater than 1 cm in diameter, while PanIN is involved in smaller pancreatic ducts less than 5 mm in diameter . Both were divided into three his-tological grades, based on their nuclear and architectural atypia namely, from PanIN-lA/B, PanIN-2 to PanIN-3, and from adenoma borderline to carcinoma of IPMN, respectively. Among them, IPMN adenoma and PanIN-lA are the lowest grades of these entities. Their exact definitions are described as follows:
PanIN-lA: These are flat epithelial lesions composed of tall columnar cells with basally located nuclei and abundant supranuclear mucin. The nuclei are small and round to oval in shape. When oval, the nuclei are oriented perpendicular to the basement membrane. It is recognized that there may be considerable histological overlap between non-neoplastic, flat, hyperplastic lesions and flat, neoplastic lesions without atypia. Therefore, some may choose to designate three entities with the modifier term 'lesion' ('PanIN/L-1A') to acknowledge that the neoplastic nature of many cases of PanIN-lA has not been unambiguously established .
IPMN adenoma: The epithelium is comprised of tall columnar mucin-containing cells that show slight or no dysplasia; i.e. the epithelium maintains a high degree of differentiation of adenomas .
As the above shows, it was overlooked, even in the context of the definition of each disease, that these low-grade neoplasias might be confused with non-neopalastic hyperplastic lesions.
Fig. 1. Hyperplastic lesion in IPMN shows either the papillary part or the flat part by HE (a). Both parts reveal negative staining for MUC1 (b) and MUC2 (c).
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