Large duct type invasive adenocarcinoma of the pancreas with microcystic and papillary patterns: a potential microscopic mimic of non-invasive ductal neoplasia.

A morphological variant of pancreatic ductal adenocarcinoma forming large ductal elements, large duct type ductal adenocarcinoma, is documented and its clinicopathological features are studied. These tumors may have microcystic and papillary growth patterns that closely mimic the non-invasive cystic and papillary pancreatic tumors such as: intraductal papillary-mucinous neoplasia, including the oncocytic variant, mucinous cystic neoplasms, and ducts involved by pancreatic intraepithelial neoplasia. In a review of 230 pancreatectomy specimens with ductal adenocarcinoma, 28 (8%) cases of large duct type ductal adenocarcinomas were identified according to following criteria: more than 50% of the tumor sections available for examination contained infiltrative ducts with a diameter larger than 0.5 mm or had a macroscopically identifiable microcystic pattern. Overall characteristics of large duct type ductal adenocarcinomas were not too different than those of conventional ductal adenocarcinomas, except that there was a slight female predominance in the former (F/M=2.3). The mean age was 67 (vs 63 in conventional ductal adenocarcinomas; P=0.015), and occurrence in the tail was slightly more common (40% vs 18% in conventional ductal adenocarcinomas; P=0.006). Grossly, cysts measuring up to 1 cm was noted in 10 cases. Microscopically, large duct type adenocarcinomas were characterized by irregularly distributed large ducts with jagged edges, lined by columnar mucinous cells often having deceptively bland cytological features and variable degrees of papillomatosis. Stromal desmoplasia had a hypercellular quality (morphologically distinct from ovarian-like stroma) in four cases, and had a myxoid quality in others. KRAS oncogene mutation was identified in 9 out of 11 cases. Median, 1-year and 2-year survival rates were 16 months, 77% and 30%, respectively, as opposed to 12 months, 52% and 30%, respectively, in conventional ductal adenocarcinoma. In conclusion, it should be recognized that, some (8%) pancreatic ductal adenocarcinomas exhibit a large duct pattern that may microscopically mimic non-invasive pancreatic tumors characterized by cystic and papillary patterns. They may be distinguished by the relatively smaller size of the cysts, irregularity of the duct contours, clustering of the ducts, presence of intraluminal neutrophils and granular debris, degree of cytological pleomorphism, and myxoid quality of the stroma. Clinical behavior appears to be slightly better than that of conventional ductal adenocarcinoma, which may be accounted by the well-differentiated nature of these tumors.