Molecular biomarkers in thyroid FNA samples.

In this issue of the JCEM, Walsh et al. (1) report the analytical validation of a molecular diagnostic method for indeterminate fine-needle aspiration (FNA) thyroid samples. Together with previous reports describing clinical performance and cost effectiveness of this method, 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 whether thyroid nodules are benign or malignant (6). Four major types of thyroid malignancy are recognized, including papillary (PTC), follicular (FTC), anaplastic, and medullary carcinomas (7). PTC, particularly follicular-variant PTC, and FTC can be difficult to differentiate with respect 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% of being malignant. Indeterminate lesions include: 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 indeterminate result undergo diagnostic surgical resection of the thyroid, with the risk of performing unneeded operations if a lesion is revealed to be benign (9). Therefore, methods are urgently needed to guide clinical decisions, in adjunct to cytology, in the case of indeterminate FNA samples, to reduce surgery-associated morbidity and additional costs. Understanding molecular pathogenesis of thyroid nodules has great potential to discover novel genetic biomarkers that might guide the decision for surgery. Recently, two approaches based on nucleic acids determination have been proposed to improve FNA accuracy: 1) detection of cancer-driving oncogene mutations to identify cancer; and 2)gene expressionprofiling, aimedat exploiting transcriptional 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, FNAB determination of BRAF V600E mutation and 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% of the cases) can also be associated to benign adenomas. Thus, the presence of RAS and PAX8/PPAR mutations in adenomas may generate falsepositive results (7). However, adenomas carrying either RAS or PAX8/PPAR genetic alterations are considered precancerous lesions (11, 14), and precautional thyroidectomy in these patients would be a reasonable choice. In the largest prospective trial so far reported, mutation-positive lesions with indeterminate cytology (i.e. atypia of undetermined significance/follicular lesion of undetermined significance, follicular neoplasm/suspicious for a follicular neoplasm, and suspicious for malignancy) re-