Molecular Testing of Thyroid Fine-Needle Aspirations Improves Presurgical Diagnosis and Supports the Histologic Identification of Minimally Invasive Follicular Thyroid Carcinomas

Background: Previous studies detecting mutations in thyroid nodule fine-needle aspiration (FNA) material differed with respect to the cytologic grading applied to the FNAs, the type of FNA material used, and the prevalence of mutations observed in the samples. Therefore, the aim of the present study was to investigate these differences as possible reasons for the discrepant sensitivities and specificities reported for the "ruling-in" approach between the previous studies. Methods: RNA and DNA was extracted from 347 routine air-dried FNA smears with available histology. PAX8/PPARG and RET/PTC rearrangements were detected by real-time quantitative polymerase chain reaction, while BRAF and RAS mutations were detected by pyrosequencing. Results: BRAF mutations were associated with a carcinoma in 100% of samples; RAS mutations were associated with a carcinoma in 57% of samples. Forty-nine percent of the carcinomas were identified by molecular testing in the group of follicular lesions, which increased the sensitivity from 60% to 80% compared to cytologic FNA evaluation alone. While follicular lesion FNAs had a 28% risk of malignancy, the risk increased to 71% for mutation-positive follicular lesions, and decreased to 18% for mutation-negative follicular lesions. Conclusion: Molecular testing of air-dried FNA samples improves presurgical diagnosis. Discrepant sensitivities and specificities reported previously are most likely related to the use of different grading schemes resulting in different compositions of the cytologic categories, interobserver variability to diagnose follicular variant of papillary thyroid carcinomas and a different prevalence of RAS mutations in follicular carcinomas. The knowledge of the molecular testing might support the histologic identification of minimally invasive follicular carcinomas.