Answer: Infiltrating mucinous carcinoma arising in intraductal papillary mucinous neoplasm

Histology: This specimen shows a papillary proliferation within the native duct system of the pancreas. The papillary projections have a focally cribriforming structure with a loss of cellular polarity. The neoplasm involves a large duct and surrounding smaller ducts. These features are diagnostic of an intraductal papillary mucinous neoplasm of the pancreas showing high-grade dysplasia (carcinoma in situ). In addition to this intraductal component, this neoplasm shows extravasated pools of mucin in which float strips of atypical epithelium. These areas are diagnostic of infiltrating mucinous carcinoma. Thus, this case represents infiltrating mucinous carcinoma arising in association with an intraductal papillary mucinous neoplasm showing high-grade dysplasia.

Discussion: Intraductal papillary mucinous neoplasms (IPMN) of the pancreas are recently described neoplasms of the pancreas. They are often confused with another group of pancreatic neoplasms, mucinous cystic neoplasms. Intraductal papillary mucinous neoplasms of the pancreas can be differentiated from mucinous cystic neoplasms based on several characteristics. IPMN tends to involve the native duct system of the pancreas, while mucinous cystic neoplasm produces cysts not connected to the native duct system. Mucinous cystic neoplasms tend to have a surrounding ovarian-like stroma and are found almost exculsively in women, while the ovarian-like stroma is absent in IPMN, which occurs in both men and women. IPMN occurs more commonly in head of pancreas, while mucinous cystic neoplasm arises primarily in body and tail of pancreas. Finally, IPMN shows the classic clinical (endoscopic) feature of “mucin oozing from ampulla of Vater,” while this is not the case with mucinous cystic
neoplasm.

Just like adenomas in the colon, intraductal papillary mucinous neoplasms of the pancreas progress through stages of increasing dysplasia, if they are not removed. Thus, intraductal papillary mucinous neoplasms can be adenomas, borderline tumors, and neoplasms with high-grade dysplasia (carcinoma in situ). Some intraductal papillary mucinous neoplasms develop invasive carcinoma, just like in this case. This invasive carcinoma may be of the tubular (conventional ductal) or mucinous variety. The latter diagnosis is made when over 50% of the neoplasm consists of mucinous or colloid differentiation, where strips of (atypical) epithelium float within pools of mucin. We have just shown that virtually all invasive mucinous carcinomas, as seen in this case, arise in the setting of an intraductal papillary mucinous neoplasm. Therefore, pathologists should look carefully for an intraductal papillary mucinous neoplasm if they identify an invasive mucinous carcinoma of the pancreas. The effect on prognosis of mucinous differentiation is not yet clear, with one study showing a significantly better prognosis for these tumors versus conventional ductal adenocarcinoma of the pancreas and another showing approximately equal survival rates for these two groups.