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The simple, unilocular cyst is lined by cuboidal or columnar epithelium resembling biliary epithelium. The aspirated fluid is typically hypocellular and non-diagnostic.
The ciliated foregut cyst is unilocular and lined by respiratory-type epithelium. Ciliated columnar cells and mucus cells are seen in cytologic specimens. This benign lesion must be differentiated from bile duct cystadenoma and cystadenocarcinoma, which are usually multilocular tumors lined by mucinous epithelium.