Mesonephric-like Adenocarcinoma of the Uterine Corpus: Comparison Between Mismatch Repair Protein Immunostaining and Microsatellite Instability Testing

1785Abstract. Background/Aim: The utility of mismatch repair (MMR) immunohistochemistry (IHC) and microsatellite instability (MSI) testing in uterine mesonephric-like adenocarcinoma (MLA) has seldom been reported. This study aimed to compare MMR IHC and MSI testing in uterine MLA. Patients and Methods: We analyzed the MMR protein expression of 25 patients with MLA and compared the results with the MSI status. Results: Of the cases, one (4.0%) was initially interpreted to have a loss of mutS homolog 2 (MSH2) and mutS homolog 6 (MSH6) expression, and eight (32.0%) to have a loss of MSH6 immunoreactivity. Upon re-evaluation of the slides and repeat IHC, all cases were revealed to demonstrate at least focal (5-20%) immunoreactivity for MSH2 or MSH6. MSI testing revealed all cases to be microsatellite stable (MSS). Conclusion: Uterine MLA is an MSS/MMR-proficient tumor. We observed complete concordance between MMR IHC and MSI testing. MMR IHC should be carefully interpreted, and discordant cases must be thoroughly reviewed. Repeat IHC and confirmatory MSI testing may be beneficial in resolving uncertain cases.