Juliet,
You will get responses to this from both ends of the spectrum here - there are radically different opinions on the site.
I have cut back on sugar, as I probably could lose 10 pounds, or more, and not miss them.
I cut back the ice cream years ago, but cut back a little more with my surgery / RT, as I am a bit lactose intolerant, and with the added bowel complications of DaVinci and RT / post-RT, well, we just won't go there.
I did have friends with other cancers tell me that their oncologists were very anti-sugar in any form. They cut back to zero, lost large percentages of their total body weight, and developed other problems. My GP, Rad. Oncologist, and Uro all felt that sugar is not the best thing for you, but that a normal intake has little if anything to do with cancer. What is normal? Another question ...
I spent a half-day with the lead dietician at the hospital trying to understand if I should be doing anything differently, and in the end, she said not much, that a reasonably balanced diet is fine. She did encourage foods that were positive for digestion and bowel function, but was most concerned that I kept a stable weight. Especially during RT, they want stability. I suspect that changes which create stress and sudden weight loss are not wanted at the same time as surgery / radiation. Preparing by making healthy changes early can not hurt, but radical changes would not make sense.
Just my non-medical report of what I was told.