Molecular Biomarkers in Thyroid FNA Samples

Abstract

In this issue of the JCEM, Walsh et al. (1) report the analyticalvalidationofamoleculardiagnosticmethod for indeterminate fine-needle aspiration (FNA) thyroid samples. Together with previous reports describing clinicalperformanceandcosteffectivenessofthismethod,this study highlights the capability of such a molecular test to reduce unnecessary surgery for benign thyroid diseases (2–5). Although rarely malignant, thyroid nodules are very common. FNA cytology is routinely used to assess whetherthyroidnodulesarebenignormalignant(6).Four majortypesofthyroidmalignancyarerecognized,includingpapillary(PTC),follicular(FTC),anaplastic,andmedullarycarcinomas(7).PTC,particularlyfollicular-variant PTC,andFTCcanbedifficulttodifferentiatewithrespect to benign thyroid nodules (7). According to the National Cancer Institute, FNA results stratify thyroid nodules as benign, malignant, nondiagnostic (when the aspirate contains scant cellularity), and indeterminate. Indeterminate lesions (nearly one third of the cases) represent the most challenging conditions and feature an overall risk of 20– 30%ofbeingmalignant.Indeterminatelesionsinclude:1) atypia of undetermined significance/follicular lesions of undeterminate significance (with a malignancy risk of 5–15%); 2) follicular or Hurthle/suspicious for follicular or Hurthle neoplasms (with a malignancy risk of 15– 30%); and 3) suspicious for malignancy (with a malignancy risk of 60–75%) (8). Commonly, patients with an indeterminateresultundergodiagnosticsurgicalresection of the thyroid, with the risk of performing unneeded operations if a lesion is revealed to be benign (9). Therefore, methodsareurgentlyneededtoguideclinicaldecisions,in adjuncttocytology,inthecaseofindeterminateFNAsamples, to reduce surgery-associated morbidity and additional costs. Understandingmolecularpathogenesisofthyroidnoduleshasgreatpotentialtodiscovernovelgeneticbiomarkersthatmightguidethedecisionforsurgery.Recently,two approaches based on nucleic acids determination have been proposed to improve FNA accuracy: 1) detection of cancer-drivingoncogenemutationstoidentifycancer;and 2)geneexpressionprofiling,aimedatexploitingtranscriptional reprogramming of malignant vs. benign cells to identify benign lesions. Nikiforov et al. (10–12) first proposed systematically applying oncogene mutation detection to molecularly classify FNA samples; this approach was further pursued by other investigators (13–15). Oncogene detection was proved feasible also in routine air-dried FNA smears (16). Thus,FNABdeterminationofBRAFV600Emutationand RET/PTC (RET/PTC1 and RET/PTC3) rearrangements (commonly associated to PTC), RAS (HRAS codon 61, KRAS codons 12/13, and NRAS codon 61) mutations (commonly associated to follicular-variant PTC and FTC), and PAX8/PPAR fusion (commonly associated to FTC) was shown to improve diagnostic accuracy and enable cancer identification. One important caveat is that RAS mutations (20–40% of the cases) and PAX8/PPAR rearrangement(2–10%ofthecases)canalsobeassociated