Clinical outcomes of resected gastrointestinal stromal tumors treated with 3 years of adjuvant imatinib in different genotypes.

839 Background: Surgical resection followed by 3 years of adjuvant imatinib is standard for high-risk resectable gastrointestinal stromal tumors (GISTs) defined by modified NIH criteria. However, the impact of different genotypes in this clinical setting is not clearly defined. Methods: We conducted a single-center retrospective study of 468 patients with high-risk resectable GIST who underwent curative resection between 1994 and 2016 in Korea. Among them, 255 received 3 years of adjuvant imatinib and 213 did not. Recurrence-free survival (RFS) in different genotypes was assessed using Kaplan–Meier estimates, log-rank tests, and Cox regression models. Landmark and dynamic landmark analyses were applied to evaluate the time-varying effects of adjuvant therapy. Results: Baseline characteristics were comparable between adjuvant and no adjuvant groups. Among 468 patients ( KIT exon 11: 187 [73.3%] vs. 163 [76.5%], KIT exon 9: 36 [14.1%] vs. 22 [10.3%], wild-type [WT]: 23 [9.0%] vs. 20 [9.4%], other mutations: 9 [3.5%] vs. 8 [3.8%] in adjuvant vs. no adjuvant groups, respectively), three years of adjuvant imatinib significantly improved RFS in the overall cohort (Hazard Ratio [HR] 0.33; 95% CI, 0.27–0.42; p < 0.001). Subgroup analyses showed consistent RFS benefit in patients with KIT exon 11 (HR 0.32; 95% CI, 0.25–0.42), KIT exon 9 mutations (HR 0.32; 95% CI, 0.18–0.58), and WT GISTs (HR 0.33; 95% CI, 0.13–0.72). For other mutations, adjuvant imatinib showed a trend toward improved RFS, although without statistical significance (p=0.089). Five-year RFS rates were also higher with adjuvant imatinib across different genotypes: KIT exon 11 (59.9% vs. 22.0%, p