Your radiation oncologist may tell you to avoid taking certain supplemental antioxidant vitamins, such as vitamins C, A, D, and E, while you're having radiation therapy. These vitamins might interfere with radiation's ability to destroy cancer cells.
Here's why: Radiation works in part by creating free radicals — highly energized molecules that damage cancer cells. Free radicals in the environment can damage all cells, but in the case of radiation treatment they are focused on the cancer cells. Antioxidants help keep free radicals from forming, or neutralize them if they do form.
Because of the potential conflict between the goal of radiation therapy (to make free radicals) and the goal of antioxidants (to neutralize free radicals), it makes sense to stop taking any antioxidant supplements during radiation therapy. When radiation is finished, you can resume taking your supplements.
Throughout your treatment, do your best to eat a well-balanced diet that contains all of the vitamins you need. Vitamins that come naturally from food are unlikely to interfere with treatment.