I just completed chemo/radiation for anal cancer, so it's probably similar to what you'll be getting. I was told to avoid whole grains and raw fruits and vegetables because the radiation will make it difficult to digest those foods. That would be my only concern with the diet.
I read a lot of messages on different boards that scared me before I started treatment. The treatment was not a lot of fun and I felt lousy most of the time, but it was not as bad as I expected. Keep thinking postive thoughts! I know that's a LOT easier said than done. Best of luck to you.