Refining NTRK Fusion Detection in Papillary Thyroid Carcinoma Through Pan-TRK Immunohistochemistry and Histopathologic Features.

Abstract

NTRK fusions are rare but recurrent driver alterations in papillary thyroid carcinoma (PTC), with therapeutic significance due to the availability of targeted TRK inhibitors. Pan-TRK immunohistochemistry (IHC) provides a practical approach for the identification of NTRK fusions; however, its application and reliability in routine pathology require further exploration. This study is aimed at evaluating the diagnostic utility of pan-TRK IHC for detecting NTRK fusions in PTC, assessing its correlation with histopathologic features, and developing a diagnostic algorithm. We analyzed 107 BRAF p.V600E-negative PTC cases using pan-TRK IHC, correlating staining patterns with molecular data and histopathologic features. RNA-based targeted sequencing confirmed gene fusions. NTRK fusion-positive tumors were enriched in distinct histopathologic features, including BRAF-like PTC with predominant follicular architecture, clear cells, and secretory-like cells. Findings such as tumor cell stratification, glomeruloid structures, and papillae with subfollicle formation (microfollicles within papillary structures) were associated with both NTRK and RET fusion-positive PTCs. Correlation of pan-TRK IHC and molecular testing results identified non-specific reactivity or false positivity in 62% of pan-TRK IHC-positive PTCs, including cases with RET fusions, BRAF fusion, or no detectable fusion. However, pan-TRK IHC with high H-scores (≥ 110) was observed exclusively in cases with NTRK fusions. For cases with lower H-scores (< 110), integrating histopathologic features improved the identification of fusion-driven PTCs. While our series further supports the limitations of pan-TRK IHC, a diagnostic algorithm that combines pan-TRK IHC H-scores and histopathologic patterns improved the triaging of NTRK molecular testing of BRAF p.V600E-negative PTCs when a stepwise approach is undertaken. This study also demonstrated that TRK protein localization may vary with tumor progression and dedifferentiation.