Objectives: An oncocytic variant of pancreatic neuroendocrine tumors (PanNET) is exceedingly rare. Here we report cytomorphological features of the oncocytic variant of PanNET and discuss how to avoid diagnostic pitfalls. Study Design: A computerized search of our laboratory information system was performed over an 18-year period to identify all cytology and surgical pathology cases where a diagnosis of PanNET was made or considered in the differential diagnosis. Three cases of the oncocytic variant of PanNET were identified. Results: Endoscopic ultrasound-guided fine needle aspiration (FNA) smears showed cohesive clusters of large atypical cells with abundant eosinophilic granular cytoplasm, anisonucleosis, nuclear enlargement and overlapping, prominent nucleoli, and a relatively smooth nuclear membrane. Nuclei were round to oval with finely granular chromatin. Additional features included rare isolated cells and glandular formation. Some of these morphological features, such as anisonucleosis, nuclear enlargement, and overlapping, prominent nucleoli, are also commonly seen in the pancreatic adenocarcinoma. All these cases were misclassified by FNA as adenocarcinoma (2 cases) or suspicious for carcinoma (1 case) and were histologically confirmed to be oncocytic variants of PanNET. Conclusions: Useful salient features of the oncocytic variant of PanNET include abundant eosinophilic granular cytoplasm, finely granular chromatin, and relatively smooth nuclear membrane. The awareness of this variant will help to avoid misdiagnosis.

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