2.5 Statistical analysis
SPSS 23.0 software was used for statistical analysis. The measurement data are expressed as the mean ± standard deviation (±S). Differences in quantitative parameters between groups were assessed using a t test (for normally distributed data) or nonparametric test (for nonnormally distributed data). Categorical variables are presented as frequencies. OS estimated by the Kaplan-Meier method was compared using the log-rank test. P values lower than 0.05 were considered significant.