Utility of Molecular Testing for Identifying Targetable Molecular Alterations and Supporting a Diagnosis of Anaplastic Thyroid Carcinoma on Core Needle Biopsy

Background Molecular testing of anaplastic thyroid carcinoma (ATC) is recommended by the American Thyroid Association early in diagnosis to support a diagnosis of ATC in specimens of ambiguous morphology/immunophenotype and enable possible alternative systemic treatment options. Ideally, molecular testing should be performed on the initial diagnostic pathology sample, frequently the core needle biopsy (CNB) specimen, despite potential concerns about tissue sufficiency. This study aimed to determine the practical feasibility and treatment implications of performing molecular testing, primarily in-house next-generation sequencing (NGS), on CNB with ATC. Methods This study was a retrospective review of CNB performed within our institution in which a diagnosis of ATC was considered. Molecular and immunohistochemistry (IHC) results were reviewed and, if not initially ordered, performed on available tissue. Results In a cohort of 24 specimens, molecular testing was successfully performed on all specimens; however, tissue scarcity limited molecular and IHC analysis for 5 specimens. Mutations in ATC-associated genes were identified in all specimens, including BRAF p.V600E (58%), RAS family (21%), and TP53 (38%). NGS testing also enabled the evaluation of microsatellite instability (MSI; 0% high), tumor mutational burden (TMB; 9% high), and a number of other potential biomarkers. Although BRAF V600E IHC demonstrated 100% sensitivity and specificity after blinded expert retrospective review, 20% of initial IHC interpretations were incorrectly or ambiguously categorized, highlighting the significant inter-observer variability associated with BRAF IHC interpretation. Conclusion Molecular testing on CNB is feasible and provides broader biomarker coverage with greater consistency and less interpretative variability than IHC.