Studies have been performed on rats, showing that rats who drank green tea instead of plain water had a reduction in tumour size. One such study was published in the Journal of Cellular Biochemistry (July 2001).
It's also been noticed that among humans, breast cancer is much less common in countries where green tea is consumed regularly. Other dietary or environmental conditions may also be involved. In Japan, one study of women diagnosed with breast cancer, found that drinking 3 or more cups of green tea a day reduced the frequency of breast cancer recurrence.
Polyphenols are thought to be the specific component that fights breast cancer, as well as a number of other diseases. They are strong antioxidants, which clean out toxic free radicals from our bodies.
Some studies from 20 years ago, pointed a finger at coffee as a possible cause of breast cancer. More recent research of 60,000 Swedish women over 13 years did not reveal such a link.
References:
Green Tea Linked to Reduced Breast Cancer Risk
Green Tea May Protect Women from Breast Cancer
Green Tea Linked to Lower Breast Cancer Risk for Asian Americans