RT Journal Article SR Electronic T1 Frequency of microsatellite instability (MSI) in upper tract urothelial carcinoma: comparison of the Bethesda panel and the Idylla MSI assay in a consecutively collected, multi-institutional cohort JF Journal of Clinical Pathology JO J Clin Pathol FD BMJ Publishing Group Ltd and Association of Clinical Pathologists SP jclinpath-2021-207855 DO 10.1136/jclinpath-2021-207855 A1 Friederike Kullmann A1 Pamela L Strissel A1 Reiner Strick A1 Robert Stoehr A1 Markus Eckstein A1 Simone Bertz A1 Bernd Wullich A1 Danijel Sikic A1 Sven Wach A1 Helge Taubert A1 Peter Olbert A1 Hendrik Heers A1 María Fernanda Lara A1 Maria Luisa Macias A1 Elisa Matas-Rico A1 Maria José Lozano A1 Daniel Prieto A1 Isabel Hierro A1 Thomas van Doeveren A1 Ivan Bieche A1 Julien Masliah-Planchon A1 Romane Beaurepere A1 Joost L Boormans A1 Yves Allory A1 Bernardo Herrera-Imbroda A1 Arndt Hartmann A1 Veronika Weyerer YR 2021 UL http://jcp.bmj.com/content/early/2021/09/27/jclinpath-2021-207855.abstract AB Aims Upper tract urothelial carcinoma (UTUC) is a rare malignancy with a poor prognosis which occurs sporadically or in few cases results from a genetic disorder called Lynch syndrome. Recently, examination of microsatellite instability (MSI) has gained importance as a biomarker: MSI tumours are associated with a better response to immunomodulative therapies. Limited data are known about the prevalence of MSI in UTUC. New detection methods using the fully automated Idylla MSI Assay facilitate analysis of increased patient numbers.Methods We investigated the frequency of MSI in a multi-institutional cohort of 243 consecutively collected UTUC samples using standard methodology (Bethesda panel), along with immunohistochemistry of mismatch repair (MMR) proteins. The same tumour cohort was retested using the Idylla MSI Assay by Biocartis.Results Using standard methodology, 230/243 tumours were detected as microsatellite stable (MSS), 4/243 tumours as MSI and 9/243 samples as invalid. In comparison, the Idylla MSI Assay identified four additional tumours as MSS, equalling 234/243 tumours; 4/243 were classified as MSI and only 5/243 cases as invalid. At the immunohistochemical level, MSI results were supported in all available cases with a loss in MMR proteins. The overall concordance between the standard and the Idylla MSI Assay was 98.35%. Time to result differed between 3 hours for Idylla MSI Assay and 2 days with the standard methodology.Conclusion Our data indicate a low incidence rate of MSI tumours in patients with UTUC. Furthermore, our findings highlight that Idylla MSI Assay can be applied as an alternative method of MSI analysis for UTUC.Data are available upon reasonable request.