Objective: To validate the eighth edition of the AJCC staging system in patients with pancreatic adenocarcinoma receiving only stereotactic body radiation therapy and chemotherapy, and to propose modifications to improve prognostic accuracy.Methods: Patients with pathologically confirmed pancreatic adenocarcinoma without metastasis who were undergoing only chemoradiotherapy were included and staged according to the seventh and eighth editions of the AJCC staging system. Meanwhile,another group of stage T4 patients from the above enrollment with only portal vein involvement with or without tumor thrombi (PV ± PVTT) were retrieved for survival comparisons. Modifications were proposed according to the survival comparisons. A cohort from the SEER database was used for extal validation of the modified staging system.Results: A total of 683 patients were included. Patients with N2 or N1 but different T stages had significantly different survival outcomes according to the eighth edition. The survival of patients with PV ± PVTT was comparable to that of patients with T4 tumors. The concordance index of the seventh and eighth editions, and the modified staging system was 0.744 (95%CI: 0.718-0.769),0.750 (95%CI: 0.725-0.775), and 0.788 (95%CI: 0.762-0.813), respectively. For extal validation, the concordance index was 0.744 (95%CI: 0.718-0.770), 0.750 (95%CI: 0.724-0.776), and 0.788 (95%CI: 0.762-0.814), respectively.Conclusions: The modified staging system is suggested to have the most accurate prognostic value. Hence, PV ± PVTT should be added to the definition of T4 tumors regardless of tumor size. Patients with N2 or N1 in different T stages could be regrouped into different substages. Additionally, stage III should be subclassified into IIIA (T3N2 and T4N0) and IIIB (T4N1-2).