Lower Gastrointestinal Tract and Microsatellite Instability

Abstract

Use of immunohistochemistry in the diagnostic gastrointestinal pathology of the lower GI tract is similar to that of the upper GI tract. CK20, CK7, and CDX-2 are probably the most commonly used markers and can identify the histogenesis of the vast majority of lower GI tract carcinomas. Tumors of neuroendocrine origin are also common in the lower GI tract, and for these, CDX-2, synaptophysin, and chromogranin immunostains are very helpful. Diagnosis of tumors of mesenchymal origin is generally straightforward based on tumor histology with the help of immunohistochemistry. The differential diagnosis between appendiceal mucinous tumors and ovarian mucinous tumors occasionally could be challenging because the two share significant similarities in both tumor histology and immunophenotype. As microsatellite instability (MSI) is thought to play a role not only in tumorigenesis but also in prognosis and response to adjuvant chemotherapy regimens, MSI testing is becoming popular. In general, immunohistochemistry and PCR-based tests for MSI correlate very well. Immunohistochemical studies are approximately 90-95% sensitive for hereditary nonpolyposis colorectal cancer (HNPCC) syndrome. Finally, in conjunction with immunohistochemistry, molecular pathology may broaden its use in pathology practice as demonstrated by K-ras and BRAF mutation test in colorectal carcinomas.