Fusion-Positive Thyroid Carcinoma: From Diagnosis to Targeted Therapy.

Purpose: Neurotrophic tropomyosin receptor kinase () fusions may act as an oncogenic driver in thyroid carcinomas. Given their low frequency, clinical, pathological, and molecular data on these patients and their responses to targeted therapies are limited.

Methods: This is an observational retrospective study conducted at a single high-volume cancer center in the United States. Data were retrospectively collected from medical records.

Results: We included 65 patients (37 adult, 28 pediatric) with an fusion-positive thyroid carcinoma (24 , 41 ), of which 54 were papillary thyroid carcinomas (PTC), four poorly differentiated thyroid carcinomas (PDTC), and seven anaplastic thyroid carcinomas (ATC). In PTC, an extensive follicular growth pattern was seen in 22 (41%) patients. In adults, fusions were 3 times more frequent (nine , 28 ), whereas in pediatric patients their frequencies were similar (15 , 13 ; = .021). In patients with PDTC/ATC treated with larotrectinib, we detected four emergent solvent front mutations (three G623R, one G595R) causing resistance to drug and disease progression. Three of them (two ATC, one PDTC) received second-line selitrectinib on a clinical trial. Partial responses were seen in all three patients, but both patients with ATC progressed within a year.

Conclusion: / fusions are seen in PTC, PDTC, and ATC, and a follicular growth pattern was observed in a high proportion of cases. In patients treated with larotrectinib, solvent front mutations are the main resistance mechanism, frequently occurring in PDTC/ATC. Responses to single-agent TRK inhibitor are short-lived in patients with ATC; thus, these drugs should be used with caution in this population.