The utility of immunohistochemical testing for mismatch repair proteins in fine needle aspiration specimens of pancreatic adenocarcinoma

Introduction: Microsatellite instability (MSI) testing is recommended for all colonic and endometrial carcinomas to screen for Lynch syndrome. The role of MSI testing in pancreatic adenocarcinoma has not been well -established. Screening can be done via immunohistochemical (IHC) staining for mismatch repair (MMR) proteins (MLH1, MSH2, MSH6, PMS2). We report our experience and the clinical utility of MMR IHC on pancreatic adenocarcinomas in fine-needle aspiration (FNA) specimens. Materials and methods: We performed a retrospective review to identify all patients diagnosed with pancreatic adenocarcinoma by FNA at our institution between December 2017 and September 2019. For cases with sufficient tumor cells for testing, the MMR results and morphology were summarized, as well as corresponding clinical information, including age, clinical stage, treatment, and concurrent other cancers. Results: From December 2017 to September 2019, there were a total of 184 pancreatic FNAs with a diagnosis of adenocarcinoma. Of these 184 FNAs, 65 (35%) contained sufficient material in the cell block to perform IHC for MMR. The cell block material was collected in either RPMI or CytoLyt. Poor technical quality precluded interpretation of PMS2 in 4 cases and MSH6 in 2 cases. All other cases showed intact expression of all four proteins. Conclusions: IHC for MMR proteins can be done on specimens collected in RPMI or CytoLyt, but RPMI appears to be more reliable. None of the pancreatic adenocarcinomas in this study showed loss of MMR protein expression. Routine testing of MMR loss may not be indicated in pancreatic adenocarcinomas in the general patient population.