Do vitamins or antioxidants interfere with radiation therapy?

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

Related Questions

Are things like radiation therapy, 5-fluorouracil, or interferon (IFN) therapy safe hypertrophic scar treatments or over the top? Other options?

Scar treatment. Nothing is "over the top", if you have done the appropriate therapies and aren't getting the results you want. You should always consider the risk/benefit ratio for anything you contemplate doing.

Is there a reason I can't take antioxidants while I'm getting radiation therapy?

Yes. Radiation takes advantage of free radicals formed in the field of radiation to cause dna damage targeted at cancer cells. Antioxidants are free radical scavengers, so they can potentially reduce the effectiveness of radiation treatments. It is best to avoid mega doses of antioxidants in the diet during and up to 6 weeks after radiation treatments.
Best not to take. Radiation works by creating free radicals. Antioxidants get rid of free radicals which has a protective effect that we want normally. But during radiation we create these in the area of the cancer to destroy cancer so we don't want to diminish this effect. All this is based on theory and radiation is so powerful that antioxidant are not likely to prevent this damage.