Malignant transformation of choledochal cyst 38 years after cystoduodenostomy mimicking Lemmel's syndrome: a rare diagnostic pitfall.

Malignant transformation of choledochal cysts after internal drainage is a known risk, but presentation nearly four decades later is exceptional. We report a case of cholangiocarcinoma developing 38 years after cystoduodenostomy in a 39-year-old female. Diagnosis was confounded by a periampullary diverticulum, mimicking Lemmel's syndrome. Multimodal imaging suggested a hilar mass, but endoscopy revealed a large diverticulum. Exploratory laparotomy and frozen section analysis confirmed adenocarcinoma arising from the remnant cyst. The patient underwent complete cyst excision, lymphadenectomy, and Roux-en-Y hepaticojejunostomy. This case underscores two critical lessons: the carcinogenic risk of internal drainage persists indefinitely, necessitating lifelong surveillance; and secondary malignancies can anatomically mimic benign conditions like Lemmel's syndrome, requiring a high index of suspicion and meticulous surgical management.