A Comparative Study of MSI and MSS Colorectal Carcinomas: Histopathological and Clinical Findings from a Single-Center Retrospective Analysis in India

Abstract Purpose Microsatellite instability (MSI) is a key molecular marker influencing colorectal cancer (CRC) prognosis and treatment response. MSI-high (MSI-H) tumors exhibit distinct clinicopathological features compared to microsatellite stable (MSS) tumors. This study compares the characteristics of MSI-H and MSS CRCs to enhance clinical understanding and management. Methods A retrospective analysis was conducted on CRC cases with confirmed MSI and MSS status. Tumor specimens were evaluated for laterality, histologic type, grade, immune response, invasion patterns, and TNM staging. MSI status was determined using immunohistochemistry (IHC) for MLH1, MSH2, MSH6, and PMS2 proteins. Statistical analysis was performed using chi-square and t-tests, with p < 0.05 considered significant. Results MSI-H tumors were predominantly right-sided (50%), with a higher frequency of mucinous histology (25%) and poor differentiation (41.7%). Intratumoral lymphocytic infiltration was higher in MSI-H tumors (33.3% vs. 14.8% in MSS, p = 0.075). MSI-H tumors had lower lymphovascular invasion (41.7%) and were more frequently diagnosed at Stage II (33.3%), while MSS tumors were more common in Stage III (66.7%). Conclusion MSI-H CRCs demonstrate right-sided predominance, mucinous histology, poor differentiation, increased immune infiltration, and lower lymphovascular invasion. These findings highlight the importance of MSI testing in prognosis and immunotherapy decisions.