Pancreatic Neuroendocrine Tumor with Acinar Pattern Mimicking Pancreatic Ductal Adenocarcinoma: A Case Report.

INTRODUCTION: Pancreatic neuroendocrine tumors (PanNETs) are rare, but their incidence has been increasing. PanNETs are usually diagnosed based on typical radiological and pathological findings. However, some exhibit unusual findings; therefore, we need to consider these and make a diagnosis through multidisciplinary evaluation. In this report, we present a case of PanNET that was challenging to diagnose preoperatively. CASE PRESENTATION: A man in his 70s with a prior diagnosis of intrahepatic cholangiocarcinoma (iCCA) was referred to our hospital (Kyoto University Hospital) for suspected recurrence of multiple intrahepatic metastases. MRI showed space-occupying lesions in the liver and a 2.7-cm hyperintense mass in the uncinate process of the pancreas. Adenocarcinoma was diagnosed by endoscopic US-guided tissue acquisition (EUS-TA) of the pancreatic lesion. Accordingly, the patient underwent neoadjuvant chemotherapy followed by pancreatoduodenectomy. However, postoperative histological and immunohistochemical evaluations of the surgically resected specimen revealed G2 PanNET with an unusual acinar structure that was not considered preoperatively. CONCLUSIONS: This case highlights the diagnostic challenge of unusual PanNETs that presented radiologically and pathologically with unusual features. A multidisciplinary evaluation and awareness of PanNETs with unusual features are essential for accurate diagnosis.