An intraductal tubulopapillary neoplasm (ITPN) is a very rare pancreatic tumor. Here we report an extremely rare case of an ITPN rupturing and causing acute peritonitis. A 50-year-old woman presented with left flank pain and vomiting. A computed tomography (CT) scan revealed gigantic multilocular cysts in the pancreatic tail and massive fluid collection in the abdominal cavity. The serum, urine, and abdominal fluid amylase levels were highly elevated, so she was conservatively treated with intraperitoneal drainage and antibiotics for a diagnosis of ruptured pancreatic cysts. After this patient recovered, a CT scan revealed a 2-cm low-density mass located in the body of the pancreas. This was diagnosed as a pancreatic ductal adenocarcinoma of the pancreatic body with an intraductal papillary mucinous neoplasm, and a distal pancreatectomy was performed. The tumor was composed of cuboidal high-grade dysplastic cells proliferating in a tubulopapillary growth pattern without mucin production. An immunohistochemical examination revealed that the tumor cells were positive for MUC1 and CK7, but negative for MUC5AC. These features led to the final diagnosis of ITPN. In this case, the solid ITPN growth obstructed the lumen of the main pancreatic duct, and the intraductal pressure of the distal side rose gradually. Then, pancreatic cysts formed and burst into the abdominal cavity when the intraductal pressure was at its maximum. However, an ITPN consists of high-grade atypical cells derived from the pancreatic ductal epithelium in principle, so the rupture may be an independent risk factor for peritonitis carcinomatosa in the future.
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