Supplementary Material for: The Use of Pan-Tropomyosin Receptor Kinase Immunohistochemistry as a Screening Tool for the Detection of Neurotrophic Tropomyosin-Related Kinase Fusions: Real-World Data from a National Multicentric Retrospective Study

<b><i>Introduction:</i></b> The neurotrophic tropomyosin-related kinase (<i>NTRK</i>) genes encode the tropomyosin receptor kinases (TRKs). Patients with solid tumors harboring an oncogenic <i>NTRK</i> fusion are eligible for treatment with TRK inhibitors. <i>NTRK</i> fusion is often associated with TRK overexpression. Pan-TRK immunohistochemistry (IHC) is used to screen for <i>NTRK</i> fusions, but immunoreactivity patterns are poorly defined. <b><i>Methods:</i></b> Data on pan-TRK immunoreactivity patterns in 2,669 solid tumors (comprising carcinomas, sarcomas, and melanocytic lesions) were retrospectively collected by nine laboratories and comprised tumor type, percentage of pan-TRK-positive tumor cells, staining intensity, cytoplasmic, membrane and/or nuclear staining pattern, and the presence or absence of <i>NTRK</i> fusion. <b><i>Results:</i></b> Overall, 2,457 tumors (92%) were pan-TRK negative and 212 neoplasms (8%) were pan-TRK positive. Twenty-two pan-TRK-positive tumors (0.8%) harbored an <i>NTRK</i> fusion, representing 10% of all pan-TRK-positive tumors. Cytoplasmic immunoreactivity was most often observed, followed by membrane immunoreactivity. Nuclear pan-TRK positivity was least frequent, but was most often (33%) associated with <i>NTRK</i> fusion. <b><i>Conclusion:</i></b> Pan-TRK IHC can be used to screen for <i>NTRK</i> fusions, especially in commonly diagnosed solid tumors with low <i>NTRK</i> fusion prevalence. In case of pan-TRK immunoreactivity, regardless of its intensity and tumor cell percentage, subsequent molecular tests should be performed to formally confirm the presence or absence of <i>NTRK</i> fusions.