Analysis of discordance between immunochemistry of mismatch repair proteins and next generation sequence testing of microsatellite instability in gastrointestinal cancers.

e16477 Background: Microsatellite Instability (MSI) is a well-established predictive biomarker for the efficacy of immune checkpoint inhibitors. MSI-high and mismatch repair deficiency (MMRd) are often used interchangeably.This study aimed to evaluate discordance between the immunohistochemistry (IHC) of mismatch repair proteins (MMR) and next generation sequencing (NGS) for MSI testing in patients with gastrointestinal cancers. Methods: This single institution retrospective study evaluated patients diagnosed with gastrointestinal cancers using an NGS database (Tempus). The cohort included individuals with stages I-IV colorectal, pancreatic, esophageal, gastric and hepatobiliary cancers, who demonstrated discordant results between MSI assessed by NGS and MMR evaluated by IHC. Primary outcome was rate of discordance between these two testing methods. Results: Between January 2013 and December 2024, a total of 548 patients with gastrointestinal cancers underwent NGS testing. Fourteen patients (2.55%) demonstrated discordance between MMR and MSI. Among these, nine patients were MMRd on IHC, with five classified as microsatellite stable (MSS) and four as indeterminate MSI on NGS. Two patients showed proficient mismatch repair (pMMR) on IHC but were identified as MSI-high by NGS. Additionally, three patients exhibited indeterminate MMR, with one showing high MSI and one classified as MSS by NGS. Within this cohort, six patients demonstrated concurrent loss of MLH1 and PMS2 proteins. Seven patients (50%) received immunotherapy, three of whom experienced disease progression during treatment. Conclusions: Recent studies evaluating the use of both IHC and NGS for MSI assessment in gastrointestinal cancers have produced conflicting results. Despite the limited sample size, our study underscores the potential for discordance between these two testing methods. Since MSI-high tumors are biologically different and respond well to immunotherapy, accurate identification using both methods is essential. However, when conflicting results arise, it remains unclear which test should take precedence, highlighting the need for more robust and reliable testing strategies. Discordant results should be carefully reviewed, and tests repeated if necessary. Further research is needed to evaluate whether patients with discordant results exhibit differential responses to immunotherapy, impacting prognosis and survival. Showing study results and discordance pattern. Type of discordance MSS MSI-H MSI-indeterminate MMRd 5 0 4 pMMR 0 2 0 Indeterminate MMR 1 1 1