2.3. Statistical analysis
Survival time was calculated from the date of diagnosis to the time of event or last follow-up. Tumor progression, relapse, occurrence of second malignancy, or death due to any cause were considered for event free survival (EFS). Overall survival (OS) was measured from the date of diagnosis to death from any cause. Patients still alive at the end of the study were censored at the date of the last observation. Survival probability was calculated by the Kaplan-Meier method and heterogeneity in survival among strata of selected variables was assessed with the Log-Rank test [14]. Five-year EFS and OS were reported along with their 95% confidence intervals (CI) as computed using the Peto-Peto method [15]. The Cox proportional hazards models were fit to compare EFS and OS while adjusting for potential confounders. Categorical variables were reported as counts and percentages and compared using Fisher’s Exact test. All data analyses were performed using the SAS statistical package (SAS, release 9.4; SAS Institute Inc., Cary, NC, USA).