Nonmucinous Biliary Epithelium Is a Frequent Finding and Is Often the Predominant Epithelial Type in Mucinous Cystic Neoplasms of the Pancreas and Liver

Mucinous cystic neoplasms (MCNs) can occur in the pancreas and liver. Classically, these cystic lesions are lined by columnar mucinous epithelium with underlying ovarian-type stroma. It has been proposed that cysts with ovarian-type stroma and nonmucinous epithelium be considered separate entities in both the pancreas and liver. Using a series of 104 pancreatic and 32 hepatic cases, we aimed to further characterize the epithelium present in MCNs. Mucinous epithelium was defined as pancreatic intraepithelial neoplasia-like columnar cells with pale pink/clear apical mucin. Epithelial cells ranging from flat to cuboidal to short columnar without obvious mucin or goblet cells were classified as nonmucinous/biliary epithelium. A mixture (at least 5%) of mucinous and nonmucinous/biliary epithelium was noted in 81%. Almost half (47%) of the cases had abundant (>50%) nonmucinous/biliary epithelium. Of the 71 cases with ≤50% nonmucinous/biliary epithelium, 8 cases demonstrated high-grade dysplasia (7 pancreas, 1 liver) and 14 demonstrated invasive adenocarcinoma (11 pancreas, 3 liver). Conversely, of the 58 cases with >50% nonmucinous/biliary epithelium, not a single case of high-grade dysplasia (P=0.007) or invasive carcinoma (P<0.001) was identified. In summary, nonmucinous/biliary epithelium frequently occurs in MCNs of the pancreas and liver. As mucinous and nonmucinous/biliary epithelia often occur together, there does not appear to be enough evidence to regard cases with predominantly nonmucinous/biliary epithelium as separate entities. Our findings suggest that mucinous change is a "progression" phenomenon in MCNs of the pancreas and liver, and only when abundant mucinous epithelium is present is there a risk of progression to malignancy.