2.5 Statistical analysis
Pairwise comparisons of the variables between groups were performed using the Mann–Whitney U test for continuous variables and Fisher’s exact test for categorical variables. A receiver operating characteristic (ROC) curve was used to identify the optimal cutoff levels that best discriminated patients in relapse. Survival functions were estimated using the Kaplan–Meier method and were compared using the log-rank test. The variables with P <0.20 by the univariate analysis were entered into a multivariate model using a Cox proportional hazards model to identify the most statistically significant parameters associated with RFS and OS. The level for a statistically significant difference was set at P <0.05 for all univariate tests. The SPSS 19.0 software package (SPSS Inc., Chicago, IL) and GraphPad Prism 5 (GraphPad Software Inc., La Jolla, CA) were used for the data analysis.