*2.5. Statistical Analyses*

An univariate Cox proportional hazard model was used to determine the predictive value of clinical parameters and [18F]FDG-PET features for OS. No analysis of histologic subtypes was performed in this heterogeneous population due to the small number of patients per subtype and previous studies reporting variable histologic subtypes to be correlated to survival. Due to the limited cohort size, not all variables were tested in the multivariate cox analysis. Therefore, multivariate Cox analysis was first performed using the 2 strongest prognostic clinical factors. Subsequently, the prognostic value of adding [18F]FDG-PET features that were significant in univariate analysis was determined for each [18F]FDG-PET feature separately. The [18F]FDG-PET features that significantly added prognostic value to clinical parameters were stratified through the median and the Kaplan–Meier method, and log-rank test were used to estimate survival for the different groups. Statistical significance was defined as *p* < 0.05. The analysis was performed with IBM SPSS v.25 (IBM Corp., Armonk, NY, USA).
