Fluorescent In Situ Hybridization Must be Preferred to pan-TRK Immunohistochemistry to Diagnose NTRK3-rearranged Gastrointestinal Stromal Tumors (GIST).

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.

Immunostaining of phospho-histone H3 and Ki-67 improves reproducibility of recurrence risk assessment of gastrointestinal stromal tumors.

Gastrointestinal stromal tumors (GISTs) are the most common non-epithelial tumors in the digestive tract. Beyond surgery, therapeutic management depends on risk of recurrence. Risk evaluation of GIST takes into account location and size of the tumor, whether or not the tumor was intact or ruptured and mitotic index. The mitotic index lacks in intra- and interobserver reproducibility. In this study, we evaluated on 61 GISTs, the reproducibility of mitotic counting using classical hematoxylin-eosin-saffron (HES) staining, and its correlation with the mitotic count obtained through immunohistochemical staining for phospho-histone H3 (PHH3) and the proliferation index based upon Ki-67 immunostaining. Mitotic counts by HES and PHH3 staining and Ki-67 proliferation index were evaluated twice by three independent observers taking into account interpretation times per tumor for each technique. HES-based and PHH3-based mitotic counts and Ki-67 proliferation index correlated well and presented good intra- and interobserver reproducibility. PHH3 staining resulted in a slight but statistically significant difference of about two more mitotic figures per 5 mm(2) than the HES-based count, which might have put some borderline tumors in a different risk category. Immunohistochemical staining for PHH3 and Ki-67 allowed more rapid interpretation than mitotic counts based upon HES staining, but only PHH3 staining allows counting of mitoses. Immunostaining using anti-PHH3 and anti-Ki-67 antibodies will eventually provide improved recurrence risk stratification of GIST and may become effective ancillary tools in deciding on optimal therapeutic management.