A case of a mucin-producing bile duct tumor diagnosed over the course of 6 years.

We report a case of a mucin-producing intraductal papillary neoplasm of the intrahepatic bile duct (M-IPNB) diagnosed over a period of 6 years. A 64-year-old man underwent follow-up evaluations for an abdominal aortic aneurysm at our hospital. In 2009, a computed tomography (CT) scan revealed a simple hepatic cyst in segment 3 of the liver. Annual CT scans initially showed almost no change in the size or shape of the cyst. The cystic lesion, which measured 5 cm in 2014, had increased to 11 cm by 2015, and a solid component was detected within the cyst. A biliary cystic tumor was suspected and we performed a left lateral hepatectomy. Pathological examination showed that the papillary lesion in the cyst included adenocarcinoma and adenoma components. We diagnosed M-IPNB in 2015. Identification of the solid component of the cyst, as well as an increase in cyst diameter in the image analyses, was critical for diagnosis of M-IPNB.