A practical approach to better identify NTRK 1–3 fusion-positive mesenchymal neoplasms by pan-Trk immunohistochemistry

Pan-Trk immunohistochemistry has become an affordable screening tool for tumors harboring NTRK1 - 3-rearrangements. However, false positive staining has been addressed especially in tumors with mesenchymal origin. As a positive staining triggers reflex testing, a better understanding about pan-Trk immunohistochemistry in these tumors has become necessary. In this work, we extensively studied pan-Trk IHC in a large cohort of mesenchymal neoplasms using two antibody clones: EPR17341 (RTU Assay, Roche/Ventana) and A7H6R (Cell Signaling Technologies). Whole slide sections of 809 individual cases, including 35 subtypes of mesenchymal neoplasms, were analyzed by two different pan-Trk antibodies. Any positivity above background in > 1% of tumor cells was classified as positive. Positive stained cases were molecularly analyzed. The specificity of clone EPR17341 was 78% and showed 21.9% false positive staining (177/809). Forty-five percent (80/177) of the false positive stained cases harbored a non-NTRK-gene fusion. When comparing the two antibodies in mesenchymal neoplasms, clone A7H6R showed 80% less false positive stains compared to clone EPR17341. Additionally, three tumors harboring a NTRK1-fusion were newly identified (0.4%) and reclassified in our cohort. Our work showed a high false positive rate in mesenchymal neoplasms using clone EPR17341. Clone A7H6R demonstrated a higher specificity and therefore could be considered in clinical practice for screening mesenchymal tumors for NTRK1 - 3-rearrangements, eventually leading to less unnecessary reflex testing.