I have been using Oral B, Neutra-Foam, 2% (0.9 Flouride Ion), with my trays. It is a foam and easy to apply in the trays. Non burning, pH neutral, 4 minute application, prescription only. Stronger flouride is available - if you can talk your dentist into it.

The new stuff the hygenist uses in the office is a "paint on" flouride.

Dental care is going to be a lifetime issue after radiation. Your gums are going to take a beating, you will have root exposure and complacency can be a dangerous risk. There is no esceape from this, unless you have dentures of course or want to gamble with Osteradionecrosis.

Flouride can be toxic if ingested. Unless you're a male between 6 and 8 years old, there doesn't appear to be any cancer risk. This has been a controversial subject since municipalities started doing it.

See http://www.webmd.com/cancer/news/20060406/does-fluoridation-up-bone-cancer-risk