Well avoiding refined sugar as much as possible is good in and of itself for myriad reasons. It was extremely frustrating for my wife and I to be told by our CCC dietician that I could always fall back on ice cream for calories during TX but understandable since I lived on Ensure Plus which is just a chemical variant of sugar.

I left my regular dentist of over 20 years on the advice of my CCC to go to prothodontist whose entire practice is oral cancer patients. His experience is that many dentist do not understand the need for flouride trays to counteract radiation's radical upsetting of the normal balance of a tooth's back-and-forth demineralization and remineralization between the tooth and saliva. So yes, I would insist on flouride trays when you see her next month. Just my opinion.

It turns out that even without eating, I still have bacteria in my mouth and the canned prescription nutrient - Jevity 1.5 - has so much sugar in it that just a little burp brings up enough to start the dental bacteria munching away at my teeth.
However my plaque is minimal and I only have to brush once a day.

Unfortunately your dentist is on the right track about the increased risk of cavities and sugar is not going to help your fight to keep your teeth.
Charm