Statistical analysis
The receiver operating characteristic (ROC) curves were used to identify the potential cut-off values of preoperative serum CA19-9 in predicting survival. The ROC curves were also used to compare the area under curves (AUC) of the modified TNM(mTNM) staging system and the eighth edition of the TNM staging system to show the advantage of each in predicting survival. The OS curves were calculated using the Kaplan-Meier method based on the length of time between the primary surgical treatment and final follow up or death.The log-rank test was used to assess significant differences between curves. Independent prognostic factors were identified by the COX proportional hazard regression model which were used to measure homogeneity and discriminatory ability. P<0.050 (bilateral) was considered statistically significant. The statistical analysis was performed using the statistical analysis program package IBM SPSS 23.0 and MedCalc v.19.6.1.