Vitamin C is a water-soluble vitamin found in leafy green vegetables and citrus fruits. It is considered to be an antioxidant, which prevents cancer by scavenging for oxygen free radicals that can cause cancer by damaging cellular DNA.
One article (1) looked at the use of vitamin C and cancer prevention. It noted that vitamin C is being studied as a cancer preventative because of its antioxidant properties. It noted that the scientific community is ignoring the benefits of vitamin C, even though there is good evidence to suggest that it can prevent pancreatic cancer, stomach cancer, and cancers of the mouth.
The article cited research done by Linus Pauling on patients given vitamin C to treat cancer. All the patients who took the placebo drug died during the study; however, there were actual remissions from cancer in a few patients who took the vitamin. It appears that vitamin C can both prevent and treat cancer.
Source: 1.Scientific proof. You can prevent cancer with vitamin C. http://www.naturalhealth365.com/scientific-proof-prevent-cancer-vitamin-c-1275.html. Accessed 9/13/16.