Scientific research provides evidence that green tea is effective in fighting cancer. 

Authors of "Why drinking green tea could prevent cancer suggest that consumption of green tea may help in preventing cancers in humans & that this has been shown in animal studies using green, and not black, tea (http://adsabs.harvard.edu/abs/1997Natur.387..561J).

But the results of human studies of the cancer preventive effects of greeen tea have been inconclusive (http://www.cancer.gov/cancertopics/factsheet/prevention/tea).

One study involving 60 men who had high-grade prostatic intraepithelial neoplasia showed that after one year of half the subjects taking a green tea supplement, only 1 incicent of prostate cancer was detected. The other 30 men in the study, who took a placebo instead of green tea, had 9 cancers in 30 men (http://www.cancer.gov/cancertopics/factsheet/prevention/tea).

As has been said, there still isn't conclusive evidence that green tea is effective as a medicine to prevent cancer, but the results of many studies done with green tea do look hopeful in providing evidence that green tea can prevent cancer.