Prevalence of MSI ‐H/ dMMR Colorectal Cancer in Japan: Data From the Clinical Study Group of the University of Osaka‐Colorectal Registry

Objectives Microsatellite instability‐high (MSI‐H) or mismatch repair‐deficient (dMMR) colorectal cancer (CRC) is a distinct subtype. Although immune checkpoint inhibitors are used for advanced cases, data on its frequency and characteristics, especially in resectable Asian CRC, remain limited. This study investigated MSI‐H/dMMR frequency and features in Japanese patients to support future personalized treatment strategies. Methods This multicenter observational study included 1464 patients with pathologically confirmed primary CRC enrolled from April 2024 to March 2025 at 25 institutions affiliated with Osaka University. MSI/MMR status and clinicopathological data were extracted from a prospectively maintained database. Univariate analyses were performed to evaluate factors associated with MSI‐H/dMMR. Results Among 1464 patients, 137 (9.4%) had MSI‐H/dMMR tumors. MSI/MMR testing was conducted preoperatively in 772 patients (52.7%). MSI‐H/dMMR was more frequent in patients aged ≥ 70 years, in females (OR = 1.46, p = 0.036), and in tumors located in the cecum, ascending, or transverse colon ( p < 0.001). It was significantly less common in Stage IV disease (OR = 0.22, p < 0.001) and KRAS ‐mutant tumors (OR = 0.22, p < 0.001), but strongly associated with BRAF V600E mutation (OR = 52.41, p < 0.001), poorly differentiated adenocarcinoma (OR = 7.17, p < 0.001), and mucinous adenocarcinoma (OR = 3.10, p = 0.002). Conclusions MSI‐H/dMMR CRC accounted for 9.4% of Japanese patients, including resectable cases, and exhibited distinct clinical and molecular characteristics. These findings provide a basis for future personalized treatment strategies in this population.