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Pooling four trials showed that microsatellite-unstable gastric cancers have a much better prognosis after surgery and gain nothing from perioperative or adjuvant chemotherapy, which may even harm them.
Individual patient data meta-analysis of 1,556 patients from four randomised trials of resectable gastric cancer (MAGIC, CLASSIC, ARTIST, ITACA-S), of whom 7.8 percent had microsatellite instability-high tumours.
Five-year disease-free survival was 71.8 versus 52.3 percent and overall survival 77.5 versus 59.3 percent for microsatellite-unstable versus stable tumours; chemotherapy improved survival in stable tumours (hazard ratio 0.65) but not in unstable ones (hazard ratio 1.03), with a suggestion of harm.
Microsatellite instability testing is recommended before perioperative chemotherapy for gastric cancer, and immunotherapy-based trials are the preferred approach for unstable tumours.