Not all NTRK fusions in mesenchymal neoplasia are driver events: implications on classification and targeted therapy.

NTRK fusions drive the pathogenesis of a distinctive group of mesenchymal neoplasms with significant impact on classification and targeted therapy. However, unexpected NTRK fusions have been reported in other sarcoma entities, raising uncertainty over their specificity and clinical management. Herein, we investigate the incidence and structural variants of NTRK fusions among a large clinicopathologic and molecular sarcoma cohort. The goal was to distinguish primary driver NTRK fusions from potential passenger events and correlate with sarcoma histotypes. NTRK1-3 fusions were queried across a large spectrum of sarcomas, profiled by targeted DNA and/or RNAseq. Fusions were classified as oncogenic when NTRK was the 3' partner, in-frame, retained kinase domain (KD), and/or RNAseq confirmation; fusions of uncertain significance (FUS) when out-of-frame, lacking full KD, and/or RNAseq negative. We identified 48 cases with NTRK fusions, detected either by pathologist-initiated RNAseq for diagnosis (n = 27) or by clinician-initiated DNAseq for therapeutic target discovery (n = 21). For the latter subset, reflex RNAseq was activated for confirmation. Integrated review confirmed 33 (69%) oncogenic fusions. Remaining were FUS, apart from one indeterminate. In all except three cases, oncogenic fusions occurred in canonical NTRK-driven histotypes. In contrast, NTRK FUS were detected in various pathologic entities, including well-differentiated/dedifferentiated liposarcoma (n = 9, 60%), two osteosarcoma, and single cases of other subtypes. NTRK1 fusions were the most common in both oncogenic and FUS groups (61%, 67%). CDKN2A/B deletions were observed mostly in oncogenic NTRK1 fusions (71%), while MDM2/CDK4 amplifications in the NTRK1 FUS. The FUS group demonstrated lower NTRK mRNA expression, with a mean of -4.60, p < 0.001. Pan-TRK immunohistochemistry was positive in cases with oncogenic NTRK fusions, and negative in FUS cases. Only two-thirds of NTRK fusions detected were functional drivers, mostly from pathologist-driven testing of suggestive histotypes. In contrast, genomic profiling in complex sarcomas often yields passenger FUS, lacking functional impact.