You may have seen this, but thought I'd post it anyway:

Conclusions: High levels of circulating testosterone and low levels of SHBG—both within normal endogenous ranges—are associated with increased risks of prostate cancer. Low levels of circulating estradiol may represent an additional risk factor. Circulating levels of DHT and AAG do not appear to be strongly related to prostate cancer risk. [J Natl Cancer Inst 1996;88:1118–26]

http://jnci.oxfordjournals.org/content/88/16/1118.short

I thought I had seen the opposite conclusion somewhere, that high levels of estradiol were associated with prostate cancer, but this indicates the opposite to be true.