*2.4. Statistical Analysis*

MedCalc (Belgium) was used as software for statistical analysis. Categorical variables were expressed as simple and relative frequencies; the numeric variables were expressed as mean, standard deviation, minimum and maximum.

The receiver operating characteristic (ROC) curve analysis was carried out to find the best threshold point for each metabolic variable and SMI, which predict the risk of progression/relapse (Supplementary Table S1). Progression free survival (PFS) was calculated from the date of pre-treatment 2-[18F]-FDG PET/CT to the date of first disease progression, relapse, death or the date of last follow-up. Overall survival (OS) was calculated from the date of pre-treatment 2-[18F]-FDG PET/CT to the date of death from any cause or to the date of last follow-up. Survival curves were plotted according to the Kaplan–Meier method and differences between groups were analyzed by using a two-tailed log rank test. Cox regression was used to estimate the hazard ratio (HR) and its confidence interval (CI). A *p* value of < 0.05 was considered statistically significant.
