I couldn't get into the link you're giving, but the one I have here puts D3 thusly:
http://ods.od.nih.gov/factsheets/vitamind.asp#h6
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Laboratory, animal, and epidemiologic evidence suggests that vitamin D may be protective against some cancers. Epidemiologic studies suggest that a higher dietary intake of calcium and vitamin D, and/or sunlight-induced vitamin D synthesis, correlates with lower incidence of cancer [44-51].
[...]
Additional well-designed clinical trials need to be conducted to determine whether vitamin D deficiency increases cancer risk, or if an increased intake of vitamin D is protective against some cancers. Until such trials are conducted, it is premature to advise anyone to take vitamin D supplements for cancer prevention.
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Meanwhile, the more optimistic sites generally ping my quackery meter--the claims they make are absolutely beyond belief. What I'm seeing is that D3 might have some preventative value, but that's not what OP is looking for. Same goes for antioxidants--they've got preventative value, but I don't see it as a treatment.
That said, I agree with
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If there is a will, there is a way. Don't give up, ever.
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because cancer predictions are so often wrong. Each case is different and pinning down how much of a chance someone has or how much time they have is, as mentioned, an inexact science.