A resected case of pancreatic neuroendocrine neoplasm with completely cystic degeneration mimicking a mucinous cystic neoplasm: a case report and literature review.

A 63-year-old woman was referred to our hospital because a cystic lesion was detected in the pancreatic tail on computed tomography (CT) performed at another hospital. Magnetic resonance cholangiopancreatography (MRCP) showed a 20 mm-sized unilocular cyst in the pancreatic tail; however no solid component was observed, then the patient was referred for follow-up observation. Six months later, MRCP showed an enlarged cyst, then contrast CT and endoscopic ultrasonography (EUS) were performed. Contrast-enhanced EUS showed a contrasted capsule and septum with internal hemorrhage. The patient was referred to surgery with a preoperative diagnosis of mucinous cystic neoplasm (MCN) with intracystic hemorrhage from a single, round, tail cyst, then robotic distal pancreatectomy was performed. Based on histopathological findings including immunostaining, the diagnosis of pancreatic neuroendocrine tumor (PNET G1) was made. Cystic neuroendocrine neoplasms of the pancreas are extremely rare, especially when they occur in the pancreatic tail as a cystic lesion mimicking an MCN, making preoperative differentiation extremely difficult. Such tumors should therefore be considered as a differential diagnosis from other cystic pancreatic tumors.