A piece of cake once in a while is going to be fine. Your dentist is just warning you that after radiation, it's super easy to get dental cavities for lots of reasons.
The key to keeping your teeth is "meticulous oral hygiene" as my prothodontist preaches. That means multiple brushings, floss and rinse daily plus most of us here at OCF also have flouride trays to put into our mouths and soak our teeth every day with a flouride gel.
Finally, yes, the Ensure and any sweet stuff can damage your teeth if you don't brush and clean it all off. I no longer have to worry so much since I only "eat" with my feeding tube but even without a smidgen of food nor a drop of any liquid in my mouth, I still need to brush and use my flouride trays according to my prothodontist.
Charm