Malignant transformation of biliary cystadenoma: a difficult diagnosis.

The case is described of a 63-year-old female with a multilocular liver cyst diagnosed as cystadenoma after imaging and fine needle aspiration. The lesion, however, proved to be an invasive cystadenocarcinoma at surgery. Cystadenoma cannot be differentiated, preoperatively, from cystadenocarcinoma and should always be considered for surgical resection.