Background: The 8^th edition of the American Joint Committee on Cancer (AJCC) staging system for gastric cancer incorporated several new changes. We aimed to assess the comparative prognostic values of the 7^th and 8^th AJCC pTNM staging systems in patients with gastric cancer (GC), and accordingly, to put forward a refined staging classification. Methods: The SEER database was queried to identify GC patients between 2004 and 2009. GC patients from Sun Yat-sen University Cancer Center (SYSUCC) were used as external validation data. The Kaplan-Meier method and Cox proportional hazards regression models were used to analyze cause-specific survival (CSS). The prognostic performance of different staging schemes was assessed using the concordance index (c-index), Akaike's information criterion (AIC), and likelihood ratio χ² test. Results: In the SEER cohort, stage migration occurred in 8.74% of patients. Survival analysis showed that it was better to treat T4bN0M0 + T4aN2M0 as stage IIIB and T4bN3bM0 as stage IV. Based on this, we established a new staging system which exhibited a superior c-index (0.7501) to the 7^th and 8^th AJCC staging systems (0.7498 and 0.7500, respectively). The new staging system also outperformed the 7^th and 8^th AJCC staging systems in terms of AIC and the likelihood ratio χ² test. The predictive superiority of the new staging system remained valid in the SYSUCC database. Conclusions: We demonstrated that some stage modifications in the 8^th AJCC pathologic staging were unnecessary. Therefore we established a new staging system, which was superior to the 7^th and 8^th staging systems.

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