The double duct sign refers to the concomitant dilatation of the common bile duct and pancreatic duct seen on MRCP (although the same can be visualized on CT, US, and ERCP). Visualization of the double duct sign typically suggests the presence of a malignancy - either and ampullary or pancreatic head carcinoma. Other malignancies with this appearance include cholangiocarcinoma of the distal common bile duct, lymphoma, metastases, and in rare cases, Kaposi's sarcoma. Primary retroperitoneal fibrosis has also been reported to cause simultaneous dilatation of both bile ducts.
However, some studies have shown that the double duct sign is not as specific a predictor of pancreatic malignancy as initially thought to be since benign entities can present similarly. The most common benign causes are chronic pancreatitis and ampullary stenosis. Plumley et al., have suggested that the specific bile duct morphology on ERCP is a better predictor of benign versus malignant causes of ductal dilatation: abrupt stenosis of a bile duct with irregular margins is more likely to indicate malignancy while gradual ductal stenosis, especially when associated with smooth margins, is more indicative of benign lesions. Other predictors of benign disease include glandular calcifications and ectasia of the branching bile ducts (seen in chronic pancreatitis).
Ahualli J. The Double Duct Sign. Radiology 2007;244:314-5.
Edge MD, Hoteit M, Patel AP. Clinical significance of main pancreatic duct dilation on computed tomography: single and double duct dilation. World J Gastroenterol 2007;31(11):1701-5.
Plumley TF, Rohrmann CA, Freeny PC, et al. Double duct sign: reassessed significance in ERCP. Am J Roentgenol 1982;138:31-5.