Clinically aggressive follicular cell-derived thyroid carcinoma: A comprehensive series with histomolecular characterization and discovery of novel gene fusions.

Thyroid cancer rates are increasing, mostly with a good prognosis and less than 2 % of cases are more aggressive. Recent efforts focus on understanding molecular events predicting tumor aggressiveness and treatment targets in advanced thyroid cancer. This study concerned 57 patients with aggressive metastatic, and/or radioiodine-refractory thyroid carcinomas, excluding anaplastic cases. Molecular profiling, including next-generation sequencing and RNA sequencing, was conducted to dissect the complex molecular landscape of these aggressive tumors. Histopathological analysis indicated that papillary carcinomas and high-grade thyroid carcinomas were predominant. The molecular analysis revealed a spectrum of mutations, with prevalent occurrences of BRAF V600E, TERT promoter, and RAS mutations. RNA sequencing identified ten gene fusions, such as NTRK and RET fusions. Three novel fusions were discovered: UGGT1::TERT, BTBD9::TERT, and TG::IGF1R, potentially driving aggressive behavior. UGGT1::TERT was linked to radioiodine-refractory tall cell PTC, BTBD9::TERT to high-grade follicular PTC, and TG::IGF1R to oncocytic carcinoma. These findings underscore the importance of TERT alterations in aggressive phenotypes and offer insights into molecular mechanisms guiding targeted therapies. Further research is necessary to confirm their significance as diagnostic and prognostic markers in thyroid cancer.