There is a clinical trial going on to determine if a daily regimen of
400 units of Vitamin E and 200 micrograms of Selenium will help avoid
prostate cancer.   There are some reasons for believing it might.  It
can't hurt after treatment as long as you don't overdo it.  (It is easy
to overdose on both of these.)

There is some reason to believe that a diet rich in calcium can
encourage prostate cancer.   In the studies, men who took in the
equivalent of four or more glasses of milk were more likely to get
prostate cancer and it was more likely to be aggressive.   But these
studies are hardly definitive.  Also, some calcium is needed for good
health so one doesn't want to go too far in the other direction.  Walsh
says there is some reason to believe that this has something to do with
Vitamin D metabolism, and eating lots of fruit may compensate for extra
calcium.