You want a good healthy diet. Cutting out any whole categories of foods as some people do tends to take you away from this. There's some evidence polyphenols are good for prostate cancer, although I might be tempted to skip taking such anti-oxidants during radiotherapy, since the whole purpose of radiotherapy is to generate the free radicals to corrupt DNA.
You may find you need to change diet during radiotherapy. For most patients, they find they need to reduce fibre content, but a small number find the opposite, so we were told not to change anything unless/until we found we had to. You want to try and maintain the same weight from the planning scan through to finishing radiotherapy too.
While on hormone therapy, I found that in conjunction with exercise (which should be considered as mandatory while on hormone therapy), I had to increase the proportion of protein I consumed, in order to retain more muscle mass.