I agree with the above, from reports here different hospitals ask for slightly different diets and while it isn't clear there may be real reasons in terms of what works better with the particular patient or the particular treatment.
My hospital gave me a mild laxative to ensure I emptied my rectum each morning, and that seemed to work OK without them asking me to take an enema as well - except for the planning scan which I guess is more critical. They were clearly worried about gas and wanted me to avoid fizzy drinks and green vegetables. I was told the requirement to avoid alcoholic drinks was because they were diuretics and they wanted all the tissues fully hydrated (and ensure the water drunk just before each treatment would work predictably in how full it made the bladder). No caffeine for the same reason.
I am someone who normally has a diet full of fibre, I like to enjoy texture in my food. So in practice I cut back a bit in general fibre (toast and marmalade rather than muesli for breakfast, but still brown bread as well as white), and was careful about veg (ate carrots but not greens). I didn't drink coffee, but allowed myself the occasional decaff tea. I confess to celebrating the end of each week with a modest glass of wine on the Friday, on the basis that gave two days to clear the small amount of alcohol from my body.