Assessing RET/PTC in thyroid nodule fine-needle aspirates: the FISH point of view

RET/PTC rearrangement and BRAF(V600E) mutation are the two prevalent molecular alterations associated with papillary thyroid carcinoma (PTC), and their identification is increasingly being used as an adjunct to cytology in diagnosing PTC. However, there are caveats associated with the use of the molecular approach in fine-needle aspiration (FNA), particularly for RET/PTC, that should be taken into consideration. It has been claimed that a clonal or sporadic presence of this abnormality in follicular cells can distinguish between malignant and benign nodules. Nevertheless, the most commonly used PCR-based techniques lack the capacity to quantify the number of abnormal cells. Because fluorescence in situ hybridization (FISH) is the most sensitive method for detecting gene rearrangement in a single cell, we compared results from FISH and conventional RT-PCR obtained in FNA of a large cohort of consecutive patients with suspicious nodules and investigated the feasibility of setting a FISH-FNA threshold capable of distinguishing non-clonal from clonal molecular events. For this purpose, a home brew break-apart probe, able to recognize the physical breakage of RET, was designed. While a ≥3% FISH signal for broken RET was sufficient to distinguish nodules with abnormal follicular cells, only samples with a ≥6.8% break-apart FISH signal also exhibited positive RT-PCR results. On histological analysis, all nodules meeting the ≥6.8% threshold proved to be malignant. These data corroborate the power of FISH when compared with RT-PCR in quantifying the presence of RET/PTC in FNA and validate the RT-PCR efficiency in detecting clonal RET/PTC alterations.