Catechins are also called flavan-3-oils. They are the major flavonoid compounds found in green tea. It has been researched extensively as to its ability to treat or prevent breast cancer.
One research review article (1) looked at the various studies that have been done on catechins and breast cancer. The writers wanted to know if there were any in vitro and in vivo studies on the ability of catechins to prevent or treat breast cancer.
It searched databases on cancer and catechins, and found that there have been many studies looking into breast cancer and catechins, finding that catechins were able to control breast cancer. Catechins were found to prevent breast cancer in all of the studies, suggesting that catechins could prevent cancer. Evidence does not exist that indicate that catechins could actually treat cancer, however.
Source: 1.Yiannakopoulou ECh. Effect of green tea catechins on breast carcinogenesis: a systematic review of in-vitro and in-vivo experimental studies. Eur J Cancer Prev. 2014 Mar;23(2):84-9.