Extrahepatic Biliary Mucinous Cystic Neoplasm as an Uncommon Cause of Acute Cholangitis.

Biliary mucinous cystic neoplasms (BMCNs) are rare tumors that account for less than 5% of cystic lesions in the biliary tract, predominantly affecting middle-aged women and often presenting as asymptomatic or with nonspecific symptoms. The primary treatment is surgical resection with negative margins. We report a case of grade I non-invasive BMCN that was completely extrahepatic, treated with biliary resection and hepaticojejunostomy. A 42-year-old female with a history of type 2 diabetes was referred for multiple episodes of acute cholangitis. Magnetic resonance cholangiography revealed complete occupation of the common bile duct by a multilocular lesion with thin septa extending into the left main bile duct. Biliary resection with Roux-en-Y hepaticojejunostomy was performed. Histopathological examination identified a mucinous cystic neoplasm with no evidence of cellular atypia, invasion, or malignancy. BMCNs are characterized by multilocular cysts lined by mucin-producing epithelium and a subepithelial stroma resembling ovarian tissue, suggesting hormonal dependence and an embryological origin. Preoperative diagnosis relies on imaging studies, with computed tomography being crucial for identifying the distinctive "cyst-in-cyst" appearance with septa. Complete resection is the preferred treatment due to the risk of malignancy, though recurrence remains a challenge in invasive cases. Mucinous cystic neoplasms of the liver are rare lesions, especially those that are extrahepatic, presenting diagnostic and therapeutic challenges. Continued research into this pathology is essential to optimize its management, treatment, and prognosis, particularly in cases of recurrence.