Mucinous cystic neoplasm of the liver with biliary prolapse mimicking intraductal papillary neoplasm of the bile duct: a case report.

Mucinous cystic neoplasm of the liver (MCN-L) is a rare cyst-forming epithelial neoplasm that typically lacks bile duct communication. Intraductal protrusion is uncommon and can mimic cystic intraductal papillary neoplasm of the bile duct (IPNB). A 39-year-old woman undergoing fertility treatment presented with recurrent hyperbilirubinemia and predominantly cholestatic liver test abnormalities. MRI demonstrated a 10.5-cm multilocular cystic lesion in segment 4 of the liver with downstream biliary dilatation, and internal septa were visible within the dilated bile duct on T2-weighted single-shot fast spin-echo (SSFSE) images. Contrast-enhanced computed tomography (CT) revealed enhanced cyst wall and septa, while endoscopic retrograde cholangiography revealed a smooth, oval-shaped filling defect. Endoscopic and intraductal ultrasonography indicated septate cystic components of the bile duct, and biopsy revealed mucin-producing columnar epithelium overlying the ovarian-like stroma, confirming MCN. The patient was treated via extended left hepatectomy with extrahepatic bile duct resection and reconstruction. Pathological examination revealed low-grade intraepithelial neoplasia. The patient remained recurrence-free for 20 months. This case suggests that, although biliary communication usually favors cystic IPNB over MCN-L, identification of intraductal extension of the cyst wall or septa within the dilated bile duct may provide an important clue to the diagnosis of MCN-L.