S3427 An Uncommon Presentation of a Cystic Pancreatic Neuroendocrine Tumor With Main Duct Dilatation Mimicking Intraductal Papillary Mucinous Neoplasm: A Diagnostic Pitfall

Introduction: While pancreatic neuroendocrine tumors (PNETs) are rare and typically solid, 10-20% may present as cystic, making cystic PNETs an uncommon diagnostic challenge. Of these, very few present with intraductal growth, and the ones described thus far are most typically located in the tail of the pancreas. We present a case of a cystic neuroendocrine tumor at the pancreatic body with main pancreatic duct (MPD) dilatation, radiologically mimicking an intraductal papillary mucinous neoplasm (IPMN). Case Description/Methods: A 66-year-old man presented with abdominal pain and constipation. Contrast-enhanced abdominal computed tomography revealed an incidental 1 cm cystic lesion in the pancreatic body with upstream main pancreatic duct dilation to 7 mm. Follow-up abdominal magnetic resonance imaging showed a 1.4 cm cystic lesion with 6 mm ductal dilation and no mural nodule, radiologically suspicious for mixed-type IPMN. Given the MPD dilatation, an endoscopic ultrasound was performed. Endoscopic ultrasound (EUS) identified a 2.0 × 1.4 cm cystic lesion with a solid component, prompting fine needle aspiration (FNA). Cytology revealed a well-differentiated grade 1 PNET, positive for CAM5.2 and synaptophysin, Ki-67 <3%. Gallium-68 DOTATATE PET demonstrated somatostatin receptor–positive, localized disease (Stage I: pT1, cN0, cM0). After a multidisciplinary discussion, the decision was made for a surgical resection given the solid component, main duct dilation, and interval growth on surveillance imaging. Histopathology confirmed a grade 2 NET with lymphovascular invasion and R0 resection. The patient remains under surveillance. Discussion: Cystic PNETs are rare, and cystic variants may mimic IPMN especially when associated with MPD dilation. Of cystic PNETs described in the literature, the majority are located in the tail. Computed tomography and magnetic resonance imaging are frequently used for initial evaluation of pancreatic cysts, but distinguishing cystic PNETs remains challenging. EUS-FNA plays a crucial role in diagnosis, especially when imaging reveals high-risk features. MPD dilation in PNETs is rare and may result from mass effect, or less commonly, ductal invasion. Our patient’s lesion showed interval growth, a solid component, and MPD involvement, prompting resection. Pathology showed lymphovascular invasion and borderline Ki-67, supporting the need for surgery. This case highlights the need to consider PNETs in cystic pancreatic lesion differentials and the value of multimodal assessment.