Assessing MSI Status in GI Cancers (Rectal, Gastric, and Colon Cancer): A Cross-Sectional Study

Background: Microsatellite instability (MSI), a marker of DNA mismatch repair (MMR) deficiency, plays a significant role in the prognosis and treatment of gastrointestinal (GI) cancers, including colorectal, gastric, and rectal malignancies.MSI-high (MSI-H) tumors, particularly in colorectal cancer (CRC), are associated with better prognosis and enhanced response to immunotherapy.However, data on MSI prevalence and its clinicopathological relevance across different GI cancer subtypes, especially in diverse populations, remain limited.Objective: To establish the prevalence of MSI-H among patients with colorectal cancer in our region and to underscore its potential in shaping individualized treatment approaches across various stages of disease. Methods:This cross-sectional study was conducted in the Oncology Department of Jinnah Postgraduate Medical Centre from October 2024 to March 2025.A total of 98 patients with histologically confirmed colon, rectal, or gastric cancer were included.MSI status was determined using PCR-based analysis of five mononucleotide markers and immunohistochemistry (IHC) for MMR proteins (MLH1, MSH2, MSH6, PMS2).Clinical and pathological data were collected and analyzed using appropriate statistical tests. Results:The mean age of patients was 41.2 13.1 years; 71.4% were male.Cancer types included colon (48%), rectal (47%), and gastric (5%).Clinical stages were: stage II (17.5%), stage III (37.1%), and stage IV (45.4%).MSI-H was detected in 11.2% of cases; 88.8% were microsatellite stable (MSS).Deficient MMR protein expression was found in 11.3% of patients.MSI-H was slightly more frequent in males, though the difference was not statistically significant. Conclusion:MSI-H was observed in 11.2% of GI cancers, indicating a meaningful subset of patients who may benefit from immunotherapy and genetic counseling.Routine MSI testing may improve personalized treatment strategies and patient outcomes.