Emergency ultrasonography showed a protruding tumor in the markedly dilated common bile duct of a 33-year-old Japanese woman. Magnetic resonance cholangiopancreatography also demonstrated the tumor clearly, almost as clearly as did percutaneous transhepatic cholangiography. With a diagnosis of common bile duct carcinoma associated with congenital choledochal cyst, pancreaticoduodenectomy was performed. In the resected specimen, as well as the protruding tumor, there was also a small slightly elevated lesion. Pathology examination showed adenocarcinoma limited to the fibromuscular layer in the protruding tumor, and adenocarcinoma limited to the mucosa in the elevated lesion. Prophylactic total excision of the choledochal cyst before the occurrence of malignant change is strongly recommended in patients with congenital choledochal cyst. However, in those who are reluctant to undergo the operation, periodic follow-up with ultrasonography and magnetic resonance cholangiopancreatography would be ideal to achieve early detection of malignant change.
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