“…Firstly, the main meta-analysis shown in Figure 2 in [ 1 ] combined case-control studies for different benign and malign tumors, including those of the head, but also non-Hodgkin’s lymphoma and leukaemia, and provides one meta-analytic summary of these. It is not common practice to combine different outcomes with different aetiologies in one meta-analytic summary [ 2 , 3 ] and, given the substantial heterogeneity observed, it is highly questionable if the common risk estimate for diseases with different aetiologies that Choi et al try to combine in their meta-analysis does exist (note that the arbitrary set of outcomes used by Choi et al is not the same as ‘all cancers’, which is an aggregate outcome used in meta-analyses).…”