Hi Nidge,
When I was in your position, my hospital issued a special 'low fibre diet sheet' to patients receiving RT.
You should eat only white bread, white rice, white pasta, steamed or boiled root veg, steamed chicken or fish, and bizarrely Jaffa Cakes.
You should avoid all red meat, fried food, green veg, beans, onions, wholemeal bread, brown rice and all fruit.
Sounds counterintuitive, but its only for a few weeks.
:)
Dave