Statistical Analysis
We compared patient demographics between histologic subtypes using frequencies and central tendency measures. Chi-squared tests determined differences in proportions between histologic subtypes for lymph node sampling and lymph node metastases. We calculated unadjusted survival estimates for OS by histologic subtype using Kaplan-Meier (KM) curves with log-rank statistics and Gehan-Breslow-Wilcoxon tests when hazards crossed. Multivariate Cox proportional hazard models estimated the hazard of death by lymph node sampling and lymph node metastasis statuses. Models adjusted for known prognostic indicators of OS in pediatric NRSTS patients, including tumor size and grade, neoadjuvant chemotherapy, and surgical margins. Histologic subtype significantly modified the relationship between lymph node sampling and OS, thus we stratified this regression model by histology. For all regression models, we confirmed proportional hazards by testing global goodness of fit for the correlation between Schoenfeld residuals and time. A Bonferroni corrected p-value <0.0125 was applied for multiple comparisons amongst stratified histological subtypes; otherwise p<0.05 was considered significant. All statistical analyses were performed using R v3.6.1 (R Core Team 2017).16