Background
Phase III RCT (SSGXVIII/AIO). 400 patients with KIT-positive GIST at high risk of recurrence after resection. High risk defined by modified consensus criteria: tumor >10 cm; mitotic count >10 per 50 HPF; tumor >5 cm with mitotic count >5 per 50 HPF; or tumor rupture.
Interventions and follow up
Arm A: imatinib 400mg daily for 3yr (n=200)
Arm B: imatinib 400mg daily for 1yr (n=200)
Primary endpoint: Recurrence-free survival (RFS)
mFollow up: 54mo (~4.5yr); long-term update 119mo (~10yr)
Arm B: imatinib 400mg daily for 1yr (n=200)
Primary endpoint: Recurrence-free survival (RFS)
mFollow up: 54mo (~4.5yr); long-term update 119mo (~10yr)
Results
5-yr RFS: 66% vs 48%, HR 0.46, 95%CI 0.32-0.65
5-yr OS: 92% vs 82%, HR 0.45, 95%CI 0.22-0.89
10-yr update (Joensuu H, JAMA Oncol, 2020; PMID:32469385):
5-yr RFS (update): 71% vs 53%
10-yr RFS: 53% vs 42%, HR 0.66, 95%CI 0.49-0.87, P=.003
5-yr OS (update): 92% vs 86%
10-yr OS: 79% vs 65%, HR 0.55, 95%CI 0.37-0.80, P=.004
5-yr OS: 92% vs 82%, HR 0.45, 95%CI 0.22-0.89
10-yr update (Joensuu H, JAMA Oncol, 2020; PMID:32469385):
5-yr RFS (update): 71% vs 53%
10-yr RFS: 53% vs 42%, HR 0.66, 95%CI 0.49-0.87, P=.003
5-yr OS (update): 92% vs 86%
10-yr OS: 79% vs 65%, HR 0.55, 95%CI 0.37-0.80, P=.004
Adverse events
3yr vs 1yr arm: edema 74% vs 59%; diarrhea 54% vs 44%; muscle cramps 49% vs 31%
Severity: most events grade 1-2; grade 3-4 events similar between arms
Discontinuation: more frequent in the 3-year arm, mostly for low-grade toxicity
Severity: most events grade 1-2; grade 3-4 events similar between arms
Discontinuation: more frequent in the 3-year arm, mostly for low-grade toxicity
Conclusions
3 years of adjuvant imatinib is superior to 1 year for both RFS and OS in high-risk resected GIST, with durable benefit at 10 years.
Key Limitations
Open-label; no observation-only arm so absolute benefit over no treatment not measured here; high-risk population limits generalizability to intermediate-risk disease; relapses cluster after stopping imatinib, raising the unresolved question of whether even longer (or indefinite) therapy is better; mutation-specific effects (e.g., PDGFRA D842V) not prospectively addressed.
Clinical Context
SSG XVIII is the basis for the standard 3 years of adjuvant imatinib in high-risk resected KIT-positive GIST, endorsed by ESMO and ASCO. PDGFRA D842V-mutant tumors should not receive adjuvant imatinib (intrinsic resistance). Trials of longer durations (e.g., 5 years) are ongoing.