It seems that there is contradictory evidence about the advantages of taking vitamins and anti-oxidants. This story from the OCF news section has a reasonable summary of current research.

http://www.oralcancerfoundation.org/news/story.asp?newsId=1197

The last line of the article:
"A lot of people are passionate about their vitamins," says Dr. Miller of the National Institute on Aging. "I don't know where they get it from, but it's not based on scientific evidence."

My suggestion would be to check with a medical oncologist about vitamin supplements and the general topic of chemo-prevention. - Sheldon