Introduction: Mismatch repair (MMR) protein expression has become the screening test of microsatellite instability (MSI).  The literature is limited on the expression of MMR proteins in periampullary carcinoma (PC). We studied MMR expression in PC and its correlation with histological differentiation and clinicopathological parameters. 

Methods: All consecutive PC specimens received within 3 years duration were classified as intestinal, pancreatobiliary and other types using morphological criteria and IHC (CDX2, MUC1, MUC2, CK17). MMR expression was studied using MLH1, MSH2, MSH6 and PMS2. MMR expression loss was correlated with histological parameters using chi-square test and survival analysis was done by Kaplan Meier using log-rank test.  

Results: Overall expression loss of MMR proteins was 8.3% (9.7% in intestinal, 7.4% in pancreatobiliary). The histopathological parameters were largely similar in cases with MMR loss and intact expression. Median survival was also similar between the two groups (median survival of 29 months Vs. 28 months in patients with MMR expression loss and intact expression respectively)  

Conclusion: Overall expression loss of MMR proteins was 8.3% with 9.7% in intestinal and 7.4% in pancreatobiliary type. Histological differentiation was superior to MMR expression loss in predicting prognosis in PC.