A case of mucinous cystic neoplasm of the pancreas misdiagnosed as a pancreatic pseudocyst at the initial exam and resected after a 2-year follow-up.

A 44-year-old woman was referred to our hospital because of a cystic lesion in the pancreatic body that was found by computed tomography (CT) as a result of a screening for impaired liver function after the patient presented with a high fever in 2011. Trans-abdominal ultrasonography (US) revealed a 33-mm unilocular cyst within the pancreatic body and a 5-mm hypoechoic mass in the pancreatic neck. Contrast-enhanced CT showed a slight enhancement around the cyst and a mild dilation of the main pancreatic duct, but neither septum nor nodule was detected inside. Contrast-enhanced endoscopic ultrasonography (CE-EUS) revealed a hyperechoic elevated lesion inside the cystic lesion without enhancement in the pancreatic body; CE-EUS also revealed a 5-mm homogeneous hypoechoic mass with a remarkable enhancement in the pancreatic neck with the use of Sonazoid(®) as a contrast medium. These lesions were diagnosed as a pancreatic pseudocyst and a neuroendocrine tumor (NET), respectively, and were followed up with periodic examinations. The cystic lesion showed contraction 6 months after the initial exam. However, US revealed an enlargement of the cystic lesion to 40 mm in diameter 2 years after the initial exam, and EUS showed irregular thickening of the wall with a cyst-in-cyst appearance. The diagnoses of a mucinous cystic neoplasm (MCN) and a concomitant small NET were made after a distal pancreatectomy. We herein report a rare case of MCN that showed various morphological changes over 2 years of observation.