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BACKGROUND It has been speculated that the Bethesda Classification System for thyroid fine needle aspirate (FNA) may be used to predict aggressive features among histologically proven malignancies. We sought to evaluate whether malignancies that were characterized as Bethesda category V or VI have more aggressive features than malignancies that were category III or IV. METHODS A prospectively maintained database was reviewed to identify thyroid malignancies treated at a single center from 2004–2009. Only cancers that could be definitively matched to a preoperative FNA were included. Associations between Bethesda category, patient demographics, histopathologic findings, and outcomes were examined. RESULTS Three hundred and sixty cancers were analyzed: 73(20%) were Bethesda category III or IV and 287 (80%) were category V or VI. The majority of Bethesda III and IV cancers were follicular variants of PTC (fvPTC) whereas the majority of Bethesda V and VI cancers were classic PTC (52% and 67%, respectively, p<0.01). Extrathyroidal extension (30% vs. 16%, p=0.02), lymph node metastases (50% vs. 31%, p=0.05), and multifocality (51% vs. 37%, p=0.03) were more common among Bethesda V and VI nodules. However, when Bethesda III or IV classic PTC and fvPTC were compared to Bethesda V or VI cancers of the same histologic subtype, there were no differences in any features. Recurrence and overall survival were the same in all groups. CONCLUSION Bethesda category may be helpful in predicting the most likely histologic subtype of thyroid cancer, but it does not carry any prognostic significance once the histologic diagnosis is known.

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