Proposal for the classification of epithelial atypism of pancreatic duct lesions.

It would be important to develop a system for classifying epithelial atypism of the pancreatic ducts, which would represent a "common world language" of atypism of pancreatic ductal epithelia. We have reported several papers on the classification of atypical epithelia of the pancreatic ducts, in which we propose five groups: Group 1, Normal epithelium; Group 2, Papillary hyperplasia without atypia: Group 3, Atypical hyperplasia; Group 4, Severe atypism strongly suggestive of malignancy; Group 5, Invasive carcinoma. This classification system for epithelial atypism can be used for all of the ducts and ductules of the pancreas, regardless of their diameter. In contrast, in Europe and the US, PanIN has been proposed to classify precancerous lesions of ordinary ductal carcinoma of the pancreas. With regard to epithelial atypism found in the peripheral branch duct, PanIN was reported to be a basis for analyzing the mechanism of the pathogenesis of pancreatic ductal carcinoma. Recently, IPMNs have also been histologically divided into three categories: adenoma, borderline and carcinoma. In these reports, PanINs usually involve ducts <5mm in diameter and IPMNs usually produce a lesion greater than 1 cm in diameter. However, we do not think it is a good idea for the classification of epithelial atypism to depend on the site or diameter of the pancreatic ducts, when we investigate atypical grades of pancreatic duct epithelia. Therefore, a classification system for epithelial atypism, which can be used for all of the ducts and ductules of the pancreas, regardless of their diameter, should be developed as a "common world language".