Reading between the lines: The interpretation of immunohistochemical staining for mismatch repair proteins and its effect on clinical decision making in endometrial cancer

Abstract

Objective: Up to 5% of endometrial cancers (EC) are associated with germline mutations in mismatch repair (MMR) genes. Screening for Lynch syndrome (LS) may be performed using clinical screening guidelines, microsatellite instability testing, or immunohistochemical (IHC) staining for MMR proteins (MMRP). Under current College of American Pathology (CAP) guidelines, any positive IHC MMR staining should be reported as intact expression. However, tumors may exhibit partial expression of MMRP, so-called heterogenous staining.