Surgical journey for participants (pts) in the MATTERHORN trial: A global, randomized, phase 3 study of durvalumab (D) plus 5-fluorouracil, leucovorin, oxaliplatin and docetaxel (FLOT) in resectable gastric/gastroesophageal junction (G/GEJ) adenocarcinoma.

353 Background: In MATTERHORN (NCT04592913), perioperative D + FLOT significantly improved event-free survival (EFS) vs placebo (P) + FLOT in pts with resectable G/GEJ adenocarcinoma (Janjigian et al. N Engl J Med 2025). The surgical journey for pts in MATTERHORN is described. Methods: In this global, Phase 3, double-blind study, pts with resectable G/GEJ adenocarcinoma were randomized 1:1 to D 1500 mg or P every 4 weeks (Q4W; Day 1 of each cycle) + FLOT Q2W (Days 1 and 15 of each cycle) for 4 cycles (2 cycles each neoadjuvant / adjuvant), followed by D 1500 mg or P Q4W for 10 cycles. Surgery delays, adjuvant treatment (tx) initiation delays and EFS by surgical status were assessed in the full analysis set (randomized pts; FAS); surgical morbidities and mortalities were assessed in the safety analysis set (pts with ≥1 tx dose; SAS). Results: Of 948 pts randomized to D + FLOT (n=474) or P + FLOT (n=474), 90.9% vs 90.3% attempted surgery, 10.1% vs 10.8% had a surgery delay, and 86.9% vs 84.4% completed surgery. The most common surgery delay was