Early tumor shrinkage and depth of response in patients with advanced gastric cancer: a retrospective analysis of a randomized phase III study of first-line S-1 plus oxaliplatin vs. S-1 plus cisplatin.

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We investigated early tumor shrinkage (ETS) and depth of response (DpR) using data from the G-SOX study comparing S-1 plus oxaliplatin with S-1 plus cisplatin as the first-line treatment for advanced gastric cancer (AGC). ETS was determined as % decrease in the sum of the longest diameters of the target lesions at the first evaluation of week 6 compared to baseline. DpR was the maximum % shrinkage during the study treatment. The impact of ETS (cutoff value 20%) and DpR (continuous value) on progression-free survival (PFS) and overall survival (OS) were assessed by the log-rank test and Cox regression analysis including prognostic factors obtained in the G-SOX study; ECOG performance status, baseline sum of tumor diameters, disease status (recurrent/unresectable), and histology (diffuse/intestinal). Among 685 patients enrolled in the G-SOX study, 632 patients who had the first tumor evaluation were analyzed. Patients with ETS ≥ 20% had longer PFS (median 4.5 vs. 2.8 months, p < 0.0001) and OS (median 14.8 vs. 10.5 months, p < 0.0001) than those with ETS < 20%. Adjusted hazard ratios of ETS < 20 vs. ≥ 20% were 0.606 (95% confidence interval (CI) 0.506-0.725) for PFS and 0.589 (95% CI 0.492-0.704) for OS. DpR was also significantly associated with PFS and OS (both p < 0.0001). These results were similar between the SOX and CS groups. In AGC patients receiving the first-line therapy, ETS and DpR might be predictors for PFS and OS.