Hepatobiliary langerhans cell histiocytosis mimicking Caroli syndrome and recurring in a liver allograft.

Langerhans cell histiocytosis (LCH) involving the hepatobiliary system can progress to sclerosing cholangitis and cirrhosis and may require liver transplantation (LT). We report a pediatric case in which hepatobiliary LCH was initially interpreted as Caroli syndrome and recurred in the liver allograft after LT. A 4-year-old girl presented with recurrent abdominal pain and imaging findings showing hepatomegaly, multifocal intrahepatic cystic changes, and tortuous bile duct dilatation. A prior needle biopsy supported congenital hepatic fibrosis with focal cirrhosis, and she underwent pediatric orthotopic LT. Explant pathology unexpectedly showed bile duct wall infiltration by S100+, CD1a+, and Langerin+ histiocytes, confirming LCH. Early post-transplant PET-CT showed no definite active extrahepatic disease. Eleven months after LT, while receiving tacrolimus-based immunosuppression, she was readmitted with abdominal pain, and CT, MRI, and PET-CT revealed multiple new hypodense and mildly hypermetabolic hepatic allograft lesions. Allograft biopsy confirmed recurrent LCH, and molecular testing was negative for BRAF V600E. The patient began vincristine and prednisone with Pneumocystis jirovecii prophylaxis; abdominal pain resolved and allograft function remained stable, with partial lesion regression on follow-up MRI. This case illustrates that hepatobiliary LCH may mimic Caroli syndrome before transplantation and can recur in the liver allograft, making complete explant histopathology, functional imaging surveillance, and prompt biopsy essential when new post-LT allograft lesions appear.