Type of publication:
Conference abstract
Author(s):
*Arunachalam J.; Nabeta G.; Naagendran M.S.; Hegde U.;
Citation:
Annals of Oncology. Conference: The ESMO Gastrointestinal Cancers Congress. Barcelona Spain. 36(Supplement 1) (pp S97), 2025. Date of Publication: 01 Jul 2025.
Abstract:
Background: Microsatellite instability (MSI), a molecular marker of defective DNA mismatch repair (MMR), is observed in ~7% of rectal cancers. MSI-high (MSI-H) tumors, arising from sporadic or germline MMR deficiency, are highly responsive to immune checkpoint inhibitors. We aimed to evaluate the prognostic significance of MSI in rectal cancer in the era of immunotherapy and to explore demographic disparities in survival using real-world data from the U.S. Method(s): We analyzed data from the Surveillance, Epidemiology, and End Results (SEER) cancer database for patients diagnosed with rectal cancer between 2018 and 2021. We assessed cancer-specific survival (CSS) across MSI subtypes-MSI-H, MSI-low (MSI-L), and microsatellite stable (MSS)-and evaluated survival differences by age, gender, race, and stage. Analyses were performed using R. Kaplan-Meier curves visualized survival outcomes, and group comparisons were done using the log-rank test. <br/>Result(s): Among 17,487 patients, 3.6% were MSI-H (n=637), 1.9% MSI-L (n=332), and 94.5% MSS (n=16,518). Overall 1-year and 3-year CSS were 90% and 75%, respectively. In metastatic patients, median CSS (mCSS) was 25 months, increasing to 36 months in those with MSI-H tumors. By MSI status, 1- and 3-year CSS were 90.6% and 77.7% for MSI-H, 89.9% and 74.9% for MSS, and 84.8% and 68.6% for MSI-L (p=0.0087). Racial disparities were evident: 1- and 3-year CSS were 92% and 80% in White patients, 88% and 70% in Black patients, and 90% and 75% in Hispanic patients (p<0.0001). Age impacted survival significantly: 1- and 3-year CSS were 91% and 74% in patients <65 years vs. 86% and 61% in those >=65 (p<0.0001). Females had better long-term survival than males, with 3-year CSS under 80% for both, but significantly higher in females (p<0.0001). Conclusion(s): MSI-H status is associated with improved survival, reinforcing its role as a favorable prognostic biomarker in rectal cancer and highlighting the importance of routine MSI testing to guide treatment decisions. Worse outcomes among older adults, males, and Black patients reflect persistent disparities in rectal cancer care. These findings underscore the urgent need to identify and address the drivers of these differences to ensure equitable outcomes. Legal entity responsible for the study: The authors. Funding(s): Has not received any funding. Disclosure: All authors have declared no conflicts of interest.
DOI: 10.1016/j.annonc.2025.05.261
