Background: Oxidative stress in blood is associated with gastric cancer (GC) prognosis, but the predictive value of an integrated oxidative stress score comprising multiple indicators remains uncertain.

Methods: Between 2009 and 2017, clinicopathological data from GC patients were split 7:3 for developing and validation cohorts. Using COX proportional hazards regression, we analyzed oxidative stress indicators to construct a Gastric Cancer Integrated Oxidative Stress Score (GIOSS). GIOSS and other indicators were evaluated for overall survival (OS) prediction using the AUC and C-index. Kaplan–Meier analysis assessed 5-year OS in high/low GIOSS groups, stratified by pTNM stage. A Cox proportional hazards regression model identified preoperative factors influencing OS, leading to a preoperative nomogram (preope-Nomogram). The model accuracy was assessed via the ROC, calibration curve, and decision curve analysis (DCA), validated in the validation cohort. The Kaplan–Meier analysis and log-rank tests compared survival among risk groups.

Results: The study included 3498 patients. GIOSS in the training cohort outperformed any single indicator in predicting 5-year OS (AUC=0.605; C-index=0.590). Low GIOSS values were associated with better 5-year survival, applicable across different pTNM stages (p<0.05). The preope-Nomogram, incorporating GIOSS, age, cT, and cN, aligned with the pTNM stage. The ROC, calibration curve, and DCA all demonstrated the model's excellent performance, consistent across validation and training cohorts. High GIOSS patients in pTNM II (p=0.010) and III (p<0.001) showed higher 5-year OS with adjuvant chemotherapy.

Conclusions: High GIOSS independently predicts 5-year OS in GC patients after surgery. The preope-Nomogram distinguishes patient prognosis in different pTNM stages and successfully identifies high-risk patients who would benefit from adjuvant chemotherapy.