Discordant Microsatellite Instability Status in Metachronous Primary Rectal Cancer Detected During IdyllaTM Microsatellite Instability Test Validation: A Case Report

Abstract Introduction/Objective Defects in DNA mismatch repair (MMR) can be identified by immunohistochemistry (IHC) MMR, Microsatellite Instability (MSI) status testing by PCR or by MSI testing by NGS. Discrepancies may occur between the different test methods. We describe a case of a discrepancy discovered during validation of an in-house IdyllaTM MSI test (PCR method), in which repeat testing by the external reference lab led to a definitive deficient MMR/MSI-High (MSI-H) status. Methods/Case Report A 67-year-old male presented with a new rectal mass on a follow-up colonoscopy. His past surgical history included a colorectal anastomosis secondary to a low anterior resection for pT3N0 upper rectal cancer ten years ago. He had a strong family history of colorectal cancer with his father, 3 brothers, 3 paternal uncles and 2 cousins affected. Colonoscopy revealed an adenocarcinoma 3 cm distal to the anastomosis. MRI showed a plaque-like mucosal thickening along the remaining rectum below the anastomosis. He underwent a complicated redo low anterior resection. Histopathology revealed a pT1N0 invasive moderately differentiated midrectal adenocarcinoma. Test results for an external PCR-based MSI test were microsatellite stable (MSS). However, repeat testing (requested after MSI-H status was detected by the in-house IdyllaTM MSI assay) revealed a deficient MMR status (loss of expression of MSH2 and MSH6 proteins) and MSI-H by PCR analysis. These discrepant results may have been due to low tumor cellularity and tumor heterogeneity (ie, heterogeneous pattern of MSH2/MSH6 loss on IHC). Results (if a Case Study enter NA) NA Conclusion Discrepancies in deficient MMR/MSI status between and within different test methods may be attributable to intratumoral heterogeneity and low tumor cellularity. In discordant cases, pathologists must not hesitate to perform a repeat analysis using alternate methods, as a deficient MMR/MSI-H diagnosis has crucial diagnostic, prognostic and predictive implications.