Fig. 2

The covariates used within the multivariable models fitted by each paper. This is a data microarray in which the studies run along the Y-axis and the covariates run along the X-axis. Only the factors used three or more times are shown in this figure for convenience; refer to Additional file 3: Figure S1 for a data microarray illustrating all covariates studied. Rows and columns are ordered in descending order, based on how many times each covariate was included in the multivariable models fitted by each study. Where patterns were similar between studies or covariates, those papers or covariates were placed next to each other. It is evident that very few studies included the same covariates within their models and that less than half of the studies included both Stage and Grade within those models. Interestingly, according to Additional file 3: Figure S1, the majority of studies included at least one covariate within their model that had not been included in any other study. Green = Included in the multivariable model; Red = Not included in the multivariable model. LNM = Lymph node metastasis; T = Depth of invasion; M = Metastasis; KPS score = Karnofsky Performance Status score (a measure of functional impairment); LVM = Lymphovascular metastasis