In the beginning she was able to eat anything she wanted including pepperoni, pizza, candy, soda, sausage, etc. Radiation is simply a concentrated x-ray aimed at a tumor site. It basically burns the tumors cells. It doesn't hurt when they do it unless you are having difficulty getting into position for treatment, but afterwards it can create a very bad sunburn. One thing about the burn though is that you really can't put anything on it.
Any greasy substance can deflect the beam or reduce it's effectiveness.
Not just any doc, the radiation oncologist. They first have to decide exactly where you will be treated and what dose, treatment duration, etc. Once that is decided, the need to mark you with very small tatoos so they can line up the machine the same way each time. They take your picture so they can be sure they are treating the right person, then they put you in position, usually lying down, line up the machine and turn it on. Total treatment time after the initial marking is less than 10 minutes.