Thyroid paraganglioma: A diagnostic pitfall in thyroid FNA

Background: Thyroid paragangliomas are extremely rare and often are misdiagnosed by preoperative fine-needle aspiration (FNA) because their cytologic features overlap with those of other thyroid neoplasms. The objective of this study was to review the cytomorphology in a series of thyroid paragangliomas and correlate the findings with histopathology. Methods: Five thyroid paraganglioma cases that underwent FNA were reviewed. Their clinical presentation, radiology features, cytomorphology, ancillary tests, and histopathology were analyzed. Results: All patients were women with an average age of 49 years (age range, 35-61 years) and presented with an asymptomatic, solitary thyroid nodule. Radiologically, these nodules (size range, 1.8-3.0 cm) were well circumscribed, hypoechoic, and hypervascular. FNA smears showed clusters of loosely cohesive, medium-to-large epithelioid cells with clear-to-eosinophilic and occasionally foamy cytoplasm that had indistinct cytoplasmic borders. The nuclei were round to oval with focal nuclear membrane irregularities, inconspicuous nucleoli, focal marked anisonucleosis, and occasional intranuclear pseudoinclusions. Naked nuclei, variable numbers of plasmacytoid cells, multinucleated giant cells, and sustentacular cells were present in the background along with blood vessels and lymphocytes. Cytology diagnoses were incorrect and included follicular neoplasm (n = 4) and follicular lesion of undetermined significance (n = 1). Final histopathology with immunohistochemistry revealed conventional paraganglioma (n = 3) or sclerosing paraganglioma with invasive features (n = 2). Conclusions: All thyroid paragangliomas were misdiagnosed on FNA as follicular neoplasms, in part because of the rarity of these tumors in this location and cytomorphology mimicking follicles. The absence of colloid, the presence of naked nuclei, focal marked anisonucleosis, and the presence of sustentacular cells are important cytology clues.