I never heard of it before, so I looked it up. One of the claims is that it's a anti-oxidant.
Since the whole point of radiotherapy is to generate free radicals which corrupt the DNA in the cancer cells, you might want to avoid potent anti-oxidants during radiotherapy as they're trying to do the opposite - mop up free radicals before they can do any harm. Although I rather doubt they'll have much impact on the radiotherapy, at best there's just no point, and at worse it might just be harmful.