“…The most commonly used covariates in multivariate analysis, were the following: age, sex, N-stage, T-stage, Union for International Cancer Control (UICC) clinical stage and therapy related information (e.g., radiotherapy dose, concurrent chemotherapy). Notably, the smoking status was considered as a covariate in approximately one-third of the studies [19,25,27,29,30,34,45], whereas alcohol consumption was considered only in two studies [19,29]. Eastern Cooperative Oncology Group Performance Score (ECOG) was considered as covariate only in two studies [24,36] and Charlson Comorbidity Index (CCI) were considered as covariates only in two and one studies, respectively [24].…”