Biliary Cystadenomas (Mucinous Cystic Neoplasms) are rare cystic epithelial neoplasms that occur predominantly in middle-aged women and have premalignant potential, with risk of progression to cystadenocarcinoma.  - Definition = Multiloculated cystic tumors lined by mucin-producing cuboidal or columnar epithelium, supported by ovarian-type stroma. These lesions can communicate with the biliary tree (though do not always) and carry malignant potential.   - Imaging characteristics = Appears as a large, well-defined multiloculated cystic lesion with internal septations, thickened walls, and possible mural nodules. Septa and walls may show contrast enhancement, distinguishing it from simple cysts. MRI typically shows T2 hyperintense cystic components with variable T1 signal depending on mucin or hemorrhage. Presence of enhancing septations or nodularity raises concern for malignant transformation.  - Subtypes =  - Biliary cystadenoma: benign, premalignant precursor.  - Biliary cystadenocarcinoma: malignant transformation of above.   - Treatment & Surveillance =  - Because of the risk of malignancy, complete surgical excision is recommended for all suspected cases.  - Aspiration, fenestration, or drainage should be avoided.  - No surveillance role after complete resection unless pathology is indeterminate.