The diet instructions vary enormously from one hospital to another.
We were told not to change our normal diet unless/until we had to. They don't want your body to change weight or composition from the planning scan to the end of treatment. One person who was losing weight was told to go and eat some cakes!
In practice, most of us had to change to a low or no fibre diet around 2 weeks in to keep bowels manageable. That was:
No veg except peeled potatoes, white rice.
No fruit except peeled apples, pears, bananas.
No brown or wholemeal bread/flour.
Lettuce may be OK, because it's basically got nothing in it except water. (BBC Food Program mentioned that you need to eat 5000 lettuces to make up 1 of your 5-a-day). It's even very low on fibre.
If you're on hormone therapy, and also during the radiotherapy, both of these need more protein intake than normal. I ate lots of chicken and salmon. (I tend to avoid red meat anyway.)
A friend who was vegetarian did get away fine on his normal vegetarian diet including broccoli which is usually a disaster for bowels during radiotherapy (and for many other bowel conditions). I tried some broccoli on a Friday after my radiotherapy session thinking I had until Monday and it should be passed by then, but I spent all weekend on the toilet, and had a bleeding bum from all the wiping by Monday. Live and learn, and laugh about it now, 5 years later. (I can eat a whole broccoli now without any problems.)