Technical ArticleFluorescent In Situ Hybridization Must be Preferred to pan-TRK Immunohistochemistry to Diagnose NTRK3-rearranged Gastrointestinal Stromal Tumors (GIST)Castillon, Marine MD*; Kammerer-Jacquet, Solène-Florence MD, PhD†; Cariou, Mélanie MSc‡; Costa, Sebastian MD§; Conq, Gwenael MD∥; Samaison, Laura MD¶; Douet-Guilbert, Nathalie MD, PhD#; Marcorelles, Pascale MD, PhD*; Doucet, Laurent MD*; Uguen, Arnaud MD, PhD*,**Author Information *Department of Pathology #Cytogenetics Unit, Department of Genetics, CHRU Brest ‡Registre des cancers digestifs du Finistère EA7479 SPURBO, Université de Bretagne Occidentale ∥Ouest Pathologie **Univ Brest, Inserm, CHU de Brest, LBAI, Brest †Departement of Pathology, CHU Rennes §Ouest Pathologie, Rennes ¶Ouest Pathologie, Quimper, France The authors declare no conflict of interest. Reprints: Arnaud Uguen, MD, PhD, Department of Pathology University Hospital Morvan, 5, Avenue Foch, Brest 29609, France (e-mail: [email protected]). Applied Immunohistochemistry & Molecular Morphology: September 2021 - Volume 29 - Issue 8 - p 626-634 doi: 10.1097/PAI.0000000000000933 Buy Metrics Abstract Tyrosine kinase inhibitors have revolutionized the treatment of patients with gastrointestinal stromal tumors (GISTs). Nevertheless, some GISTs do not contain any targetable KIT or PDGFRA mutations classically encountered in this field. Novel approved therapies targeting TRK chimeric proteins products of NTRK genes fusions consist in a promising approach to treat some patients with GISTs lacking any identified driver oncogenic mutation in KIT, PDGFRA or BRAF genes. Thus, an adequate testing strategy permitting to diagnose the rare NTRK-rearranged GISTs is required. In this work, we studied about the performances of pan-TRK immunohistochemistry (IHC) and NTRK1/2/3 fluorescent in situ hybridization in a series of 39 GISTs samples. Among 22 patients with GISTs lacking KIT or PDGFRA mutations, BRAFV600E IHC permitted to diagnose 2/22 (9%) BRAFV600E-mutated GISTs and, among the 20 KIT, PDGFRA, and BRAF wild type tumors, 1/20 (5%), NTRK3-rearranged tumor was diagnosed using NTRK3 fluorescent in situ hybridization. Pan-TRK IHC using EPR17341 and A7H6R clones was negative in this NTRK3-rearranged sample. Pan-TRK IHC was frequently positive in NTRK not rearranged tumors without (24 samples analyzed) or with (15 samples analyzed) KIT or PDGFRA mutations with major discrepancies between the 2 IHC clones (intraclass correlation coefficient of 0.3042). Given the new therapeutic opportunity offered by anti-TRK targeted therapies to treat patients with advanced cancers including GISTs, it is worth to extend molecular analysis to NTRK fusions testing in KIT, PDGFRA, and BRAF wild type GISTs. Pan-TRK IHC appears not relevant in this field but performing a simple NTRK3 fluorescent in situ hybridization test consists in a valuable approach to identify the rare NTRK3-rearranged GISTs treatable using anti-TRK therapies. Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.