Digital droplet PCR TERT promoter mutational screening in fine needle aspiration cytology of thyroid lesions: A highly specific technique for pre‐operative identification of high‐risk cases

Background Despite the advent of comprehensive molecular testing in surgical pathology, most centers still rely on the morphological assessment of fine‐needle aspiration cytology (FNAC) to triage patients with thyroid nodules for surgery. Subsets of patients could benefit from the inclusion of molecular testing to increase the diagnostic and/or prognostic properties of the cytology analysis, including the assessment of TERT promoter mutations, an event coupled with thyroid malignancy, and poor prognosis. Methods In this prospective study, preoperative FNAC material from 65 cases was assessed for TERT promoter hotspot mutations C228T and C250T using the digital droplet PCR (ddPCR) technique on frozen pellets and re‐evaluated postoperatively. Results Our cohort consisted of 15 B‐III (23%), 26 B‐IV (40%), 1 B‐V (2%), and 23 (35%) B‐VI lesions according to the Bethesda System for Reporting Thyroid Cytopathology. TERT promoter mutations were detected in 7 cases; 4 papillary thyroid carcinomas (all with preoperative B‐VI status), two follicular thyroid carcinomas (one B‐IV and one B‐V status), and one poorly differentiated thyroid carcinoma (with B‐VI status). All mutated cases were verified by mutational analysis of tumor tissue derived from postoperative formalin‐fixed paraffin‐embedded tissue, while all cases identified as wild‐type on FNAC remained wild‐type postoperatively. Moreover, the occurrence of a TERT promoter mutation was significantly associated with malignant disease and higher Ki‐67 proliferation indices. Conclusion In the present cohort, we found that ddPCR is a highly specific method for detecting high‐risk TERT promoter mutations on thyroid FNAC material that could guide different surgical approaches in subsets of indeterminate lesions if reproduced in larger materials.