All treatment centres seem to have different guidelines and procedures on the day.
My treatment centre told us not to change our diets unless/until there was a need to do so. They want you to try and maintain the same weight from the planning scan through to the end of the radiotherapy. (One person who was losing weight was told to go and eat some cakes!) They also never mentioned what foods might be gassy or that this is an issue, which some centres do.
I found from the second week, I had to start switching to a low-fibre diet, and by the 3rd week I was on a fibre-free diet. That's basically no fruit except peeled apples, pairs, bananas; no veg except peeled potatoes, white rice, white pasta, no brown bread/flour, i.e. the epitome of a poor diet. I still had breakfast cereal with milk - lowest fibre breakfast cereal I could find was Crunchy Nut Cornflakes, and no one mentioned any problems with milk.
You need plenty of protein because the radiotherapy causes loads of healing to be required. I was mainly eating chicken and salmon. I don't think there is a problem seasoning the chicken, but avoid smothering it in fibre.
I was also doing a lot of exercise (cycling) during my radiotherapy. I subsequently found research which suggests this improves the effectiveness of it.