Statistical analysis
Continuous variables are expressed as the mean ± SD, and the significant differences were analyzed with a Student’s t test. Categorical data are expressed as the number and percentages, and were compared using an χ2 test or Fisher’s exact test. Univariate and multivariate Cox proportional hazards regression analyses were performed on candidate variables to predict the dichotomous outcome. The variables with a P < 0.1 in the univariate analysis were input into the multivariate analysis. Survival curves were calculated using the Kaplan–Meier method and compared with the Log Rank test. A probability value of < 0.05 was deemed significant. Statistical analyses were conducted using the EZR software (Saitama Medical Center, Jichi Medical University, Saitama, Japan), which is a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria).