Is it the creatine or the anabolic androgenic steroids? Need for assessing the steroids role in testicular cancer

We have read with considerable interest the case-control study by Li et al. ( 2015 ), in which muscle building supplement (MBS) use was found as an associated factor with testicular germ cell cancer. It is important to remark that the association remained statistically significant even after controlling for important potential confounders. However, we consider that there is one non-assessed variable that might be relevant in the multi-causal model for testicular cancer.