Why should you avoid vitamins c and e when on radiation treatment?

Oxygen radicals. A portion of the activity of radiation treatment (and also chemo) is due to the local production of free oxygen radicals which take part in the destruction of the cancer. Vitamins c and e have anti-oxidant capacities and their concurrent use is considered to have some effect of lowering the production of these oxygen free radicals and therefore making the treatment less effective.
Antioxidants yes! However, a weight on the orac scale, vitamin c & e are quite low. Of note is the fact that certain spices, ie. Cinnimon, on weight basis or far more potent. So is pomegrante juice, red wine, grape, and cranberry juices. These are irrefutable laboratory studies, replicated many times!
Unresolved question. Some people believe that the antioxidant effect of vitamin c and e may reduce the effects of radiation therapy. This is not a proven effect.