Cystic intraductal papillary adenocarcinoma of the bile duct of the caudate lobe initially manifesting as a simple cyst on CT.

Cystic-form, intraductal papillary neoplasms of the bile ducts (IPNBs) usually manifest as a multilocular cystic mass or unilocular cystic mass with papillary mural nodules. To our knowledge, cystic IPNBs forming single, unilocular cystic lesions without a visible papillary tumor and mimicking a simple hepatic cyst or mucinous cystadenoma have never been reported. Here, we report a case of cystic IPNB that initially appeared as a simple cyst and subsequently became a unilocular cystic mass with a papillary mural nodule on follow-up CT.