How do we define "castration" in men on androgen deprivation therapy?

Androgen deprivation therapy (ADT) is the mainstay for the treatment of advanced prostate cancer. Since the clinical evolution from surgical orchiectomy, we have typically used ADT and orchiectomy to be synonymous terms for castration. The goal of this study is to determine if, in contemporary medical practice, surgical and chemical castration provide for similar levels of diminishment of total and free testosterone. Further, what approaches should be used to most accurately measure testosterone levels in men with advanced prostate cancer and what cutoff values, for example for total testosterone 50 ng dl^-1 or 20 ng dl^-1, should be utilized. Studies available in the literature have been analyzed and compiled to address these questions. Finally, evidence is provided that free testosterone, the biologically active component, should be utilized to provide clinically relevant state of castration.