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Background: To the authors' knowledge, the impact of specific cytologic patterns in thyroid fine-needle aspiration (FNA) for follicular and Hurthle cell carcinoma is not known.Methods: The results of all thyroid FNAs performed from 1997 through 2015 with corresponding resections of follicular and Hurthle cell carcinomas were reviewed.Results: A total of 24 follicular carcinomas and 35 Hurthle cell carcinomas were identified. All FNA specimens from follicular carcinomas had a microfollicular component (3 of which were noted only in 3-dimensional groups), but only 18 cases (75%) had cytologic atypia, which most commonly consisted of enlarged nuclei with slightly pale chromatin. Diagnosing cases without cytologic atypia as benign would significantly decrease the sensitivity of the test (P = .02). Five Hurthle cell FNA specimens consisted of only scant Hurthle cells. The remaining Hurthle cell FNA specimens could be divided into those with large cell dysplasia (15 cases), those with small cell dysplasia (10 cases), cases with anisonucleosis insufficient for a diagnosis of large cell dysplasia (4 cases), and cases with Hurthle cells in flat sheets without atypia (1 case). Diagnosing cases with flat sheets without atypia as benign did not appear to affect the sensitivity of the test (P = 1.00).Conclusions: Diagnosing FNA specimens with microfollicles without atypia as benign appears to significantly reduce the sensitivity for follicular carcinoma to 75%; in contrast, diagnosing FNA specimens with Hurthle cells in flat sheets without anisonucleosis as benign does not. Cancer Cytopathol 2016;124:729-36. © 2016 American Cancer Society. [ABSTRACT FROM AUTHOR]

Hürthle cell lesions are difficult to classify by cytology alone. Molecular companion testing has added some insight, but has not resolved these diagnostic challenges. [ABSTRACT FROM AUTHOR]