Familial adenomatous polyposis (FAP) is associated with extracolonic manifestations such as duodenal and ampullary neoplastic lesions, but biliary tract tumours have rarely been described. We present a unique case of intraductal papillary neoplasm of the bile duct (IPN-B) in a patient with FAP, supported by molecular confirmation of APC-driven tumorigenesis.

A 57-year-old female with a pathogenic germline APC variant (exon 15 c.1960C>T, p.Q654X) and prior pancreas-preserving duodenectomy presented with acute cholangitis. MRCP and ERCP revealed intraductal masses causing intra- and extrahepatic bile duct dilatation. Cholangioscopy showed villous mucosal lesions rather than mobile stones or debris. Targeted biopsies demonstrated tubulovillous adenomas with low-grade dysplasia. Following multidisciplinary discussion, the patient underwent liver transplantation combined with a Whipple procedure with curative intent. Surgical pathology confirmed IPN-B with focal high-grade dysplasia, as well as a synchronous intraductal papillary mucinous neoplasm of the pancreas with low-grade dysplasia. Molecular analysis of the biliary lesion demonstrated a second somatic APC pathogenic variant, providing evidence for a two-hit carcinogenic mechanism similar to colorectal and duodenal FAP-associated neoplasms. Six months post-operatively, the patient remains clinically well. This case highlights IPN-B as a potential, under-recognized extracolonic manifestation of FAP. Awareness of this rare manifestation may aid timely recognition in FAP patients presenting with unexplained cholangitis.