A pancreatobiliary-type carcinoma in situ at the periphery of a mural nodule developed from a gastric adenoma in an intraductal papillary mucinous neoplasm.

We report a rare case of an intraductal papillary mucinous neoplasm (IPMN) with a pancreatobiliary-type carcinoma in situ (CIS) that originated around a mural nodule formed in a gastric-type adenoma. A 64-year-old man was admitted to our hospital in December 2001 for dilation of the main pancreatic duct, detected on abdominal ultrasonography. Branch-duct-type IPMN (diameter 25 mm) was diagnosed, and because mural nodules were absent, the patient was annually followed up for 5 years. In 2006, computed tomography scans revealed thickening of the tumor wall and the development of a mural nodule (diameter approximately 6 mm); pancreatoduodenectomy with regional lymph-node dissection was performed. Histopathologic examination showed an intraductal papillary mucinous carcinoma arising from an adenoma. Hematoxylin and eosin (H&E) staining revealed that most of the tumor components, including the mural nodule, had adenomatous changes, indicating the tumor to be of the gastric type; however, immunohistochemistry showed positive MUC2 expression. Histologically, the tissues around the nodule, including those showing a cribriform pattern, were diagnosed as CIS. These tissues were classified as the pancreatobiliary-type on the basis of the results of both H&E staining and immunohistochemistry. The patient remained disease-free for 3 years after surgery.