Proposal of a molecular testing algorithm for differentiated thyroid cancer (DTC).

Abstract

e18090 Background: In the growing era of precision medicine and tumor-agnostic therapies, the molecular testing of DTC is becoming mandatory to optimize diagnostic and therapeutical strategies. Even if the most frequent oncogenic events, occurring in a mutually exclusive manner, are BRAF V600E, followed by RAS, gene fusions mainly occurring in RET, NTRK1-3, and ALK genes have been also found. In this scenario, the definition of a molecular testing algorithm to be implemented for clinical practice is needed, in order to optimize resources and time for better clinical patient management. Methods: Gene mutations and gene fusions were evaluated on DNA and RNA extracted from cytologic or histologic samples. Mutations were tested using a lab-developed multi-gene panel able to analyze the hot-spot regions of 28 genes (total of 343 amplicons, 21.77 kb); gene fusions were tested using the Oncomine Focus assay panel. Results: Molecular characterization was performed on a total of 63 DTC (50 papillary thyroid carcinoma - PTC, 8 – follicular thyroid carcinoma - FTC, 5 – hurthle cell carcinoma - HCC) by the treating clinician for diagnostic, prognostic or clinical reasons. In 27 cases (42.8%) at least one DNA mutation was detected, 17 cases (26.9%) were positive for gene fusion rearrangement, in only 2 cases (3.1%) both DNA mutations and gene rearrangements were found and in the remaining 17 (26.9%) cases no DNA mutations or rearrangements were detected. Among mutated-subgroup, BRAF V600E mutation was the most common (14 of 63 cases – 22.2%), followed by RAS mut (9 cases – 14.2%: 4 NRAS, 3 HRAS, and 3 KRAS), both as single event in 7 and 4 cases respectively. Substitutions in TERT promoter region was found in 16 cases (25.3%), of which 14 as secondary event. Other mutations in TP53 (4 cases –6.3 %, of which 2 as single event), PIK3CA (1 case – 1.5%), AKT1(1 case – 1.5%), and RNF43 (1 case – 1.5%) were detected. Among riarranged subgroup, RET fusions and NTRK1-3 was found in 8 cases respectively (12.6%), while ALK fusion was found 1 case (1.5%). The concomitant mutational and fusion events found in 2 cases were the following: 1) HRAS and PAX8/PPARG; 2) PIK3CA/TP53/TERT and RET/CCDC6. Conclusions: As expected, BRAF and RAS mutations represent the most common genetic events in DTC and occur in a mutual exclusive manner. However, in about 30% of patients, mostly of young age, with papillary histology and lymph nodal involvement, a clinically actionable gene rearrangement has been found. Thus, we propose a two-step molecular testing algorithm with a first-level test to identify the most common mutations and a second-level test, including less common mutations and gene rearrangements, if the genes analyzed with the first level test are wild-type. The second-level test may be performed up-front in cases where there are clinicopathologic features suggestive of fusion gene TC gene or in clinically aggressive unconventional cases for which molecular therapy is being considered.