The CT fish mouth ampulla sign: a highly specific finding in main duct and mixed intraductal papillary mucinous neoplasms.

Main duct and mixed intraductal papillary mucinous neoplasms (IPMN) are pre-malignant cystic pancreatic neoplasms associated with pancreatic duct dilatation. Distinguishing these from benign causes of pancreatic duct dilatation is important in order to allow appropriate surveillance or surgery. A patulous duodenal papilla with extrusion of mucus at endoscopic evaluation, the endoscopic fish mouth ampulla (E-FMA) sign, is reported in main duct and mixed IPMN. We aimed to establish whether a CT correlate (CT-FMA) of this sign exists and whether this was associated with the presence of invasion or high-grade dysplasia. We defined the CT-FMA sign as an uninterrupted column of water attenuation material running from the pancreatic duct to the duodenal lumen. A retrospective, blinded review of 44 patients with histologically confirmed IPMN and 87 age-matched controls with pancreatic duct dilatation on CT was undertaken. A case-control series matched for the degree of pancreatic duct dilatation was used to compare the rates of invasion or high-grade dysplasia between main duct and mixed IPMN patients, with and without a CT-FMA sign. The CT-FMA sign could be identified in 18.5% patients with main duct/mixed IPMN with specificity 100%, positive predictive value 100% and negative predictive value 79.8%. A significant association was found between CT-FMA in main duct/mixed IPMN compared to controls, but not with the presence of high-grade dysplasia or invasion. The CT-FMA sign is a newly reported, highly specific sign of MD and mixed IPMN. If a fish mouth ampulla is identified at CT, a diagnosis of main duct or mixed IPMN is highly likely.