Do vitamins or antioxidants interfere with radiation therapy?

Not likely: If they are used in moderate amounts according to recommended doses there should not be an issue.

Yes/No: Not when used in moderation. But avoid mega doses of antioxidanta. Also stop using vitamin E supplement. One multivitamin is ok to use.

Theoretically: Oxidation is required for radiation induced damage to the dna of cancer cells to be made permanent. Theoretically it would be advantageous to take oxidative agents, but there is the possibility that artificially elevated oxidation levels may increase normal tissue damage.