A case of intraductal oncocytic papillary neoplasm treated with distal pancreatectomy.

Introduction: Intraductal oncocytic papillary neoplasms (IOPNs) are rare pancreatic tumors that are difficult to diagnose preoperatively owing to the absence of characteristic clinical symptoms and imaging findings. We report a case of IOPN treated with distal pancreatectomy and review the relevant literature. Case presentation: A man in his 70s with a history of prostate cancer underwent annual follow-up computed tomography, which revealed diffuse pancreatic atrophy and main pancreatic duct discontinuity. Magnetic resonance imaging and endoscopic ultrasonography demonstrated a protruding tumor within the dilated main pancreatic duct. Endoscopic ultrasound -guided fine-needle aspiration suggested adenocarcinoma, raising suspicion for pancreatic ductal adenocarcinoma (PDAC). Given this suspicion, neoadjuvant chemotherapy with gemcitabine and S-1 was initiated, followed by distal pancreatectomy. Histopathological examination confirmed IOPN. The postoperative course was uneventful, with no recurrence at 1 year. Conclusion: IOPN generally has a more favorable prognosis than conventional PDAC following surgical resection, but the presence of invasion directly correlates with recurrence risk. Recurrence occurs in approximately half of invasive cases, with reported distant metastases to lymph nodes, liver, and rarely brain metastases. Re-resection is effective and contributes to long-term survival. Prognostic factors include R0 resection, normal carbohydrate antigen 19-9 levels, and the presence of lymph node metastasis. Although IOPN is often difficult to distinguish from intraductal papillary mucinous neoplasm or PDAC on imaging and histopathology, characteristic 18F-fluorodeoxyglucose positron emission tomography findings and immunohistochemical staining for MUC5AC and MUC6 are helpful for diagnosis.