Microsatellite instability and oncological outcomes in Thai patients with endometrial cancer

Here, we determined the frequency of microsatellite instability (MSI) and the impact of MSI-high (MSI-H) on clinical outcomes of Thai patients with endometrial cancer (EC). Tissue samples of 110 Thai patients with EC, who had undergone surgical staging, were tested for mismatch repair (MMR) gene deficiency, and the patients were grouped into MSI-H and MSI-stable (MSI-S) groups; 24.5% had MSI-H. Unlike MSI-S group patients, MSI-H group patients had synchronous and metachronous cancer. They showed better 3-year disease-free survival (DFS) than those in the MSI-S group (<i>p</i>=.182; 92.3% vs. 82.6%). The 3-year overall survival was 96.2% in MSI-H and 86.4% in MSI-S groups (<i>p</i>=.163). Multivariate analyses showed lower uterine involvement (<i>p</i>=.004), myometrial invasion ≥50% (<i>p</i>=.032), lymphovascular space invasion (<i>p</i><.001) and MSI-S (<i>p</i>=.006) as prognostic factors for DFS. Our study showed that the prevalence of MMR gene deficiency in Thai patients with EC is common and associated with better outcomes.Impact Statement<b>What is already known on this subject?</b> Microsatellite instability (MSI) occurs in approximately 20-40% of endometrial cancer (EC) cases. MSI analysis in EC can identify patients at higher risk of hereditary nonpolyposis colorectal cancer and those having prognostic factors. Additionally, it is predictive of immune checkpoint inhibitor treatment. However, current evidence shows a correlation between clinicopathological characteristics and EC prognosis. Studies on EC and MSI status effect on survival outcome have yielded inconsistent results regarding the pathological significance of MSI in such malignancies.<b>What do the results of this study add?</b> The prevalence of mismatch repair (MMR) gene deficiency in Thai patients with EC is common (24.5%) and associated with better outcomes.<b>What are the implications of these findings for clinical practice and/or further research?</b> This study highlights the prevalence and impact of MSI on oncological outcomes in patients with EC in a low-incidence country. Future studies should focus on the detection of germline mutation to understand the accurate prevalence of Lynch syndrome in Thai patients with EC.