First of all, what is green tea? Just like black tea, it comes from the Camellia sinensis plant. Black tea is fermented and green tea is not (oolong tea is partly fermented). Green tea is very popular in China and Japan and is gaining favour in other nations too. They have been drinking green tea for health reasons for over 5,000 years in Asia.
Some of the various chemical compounds in green tea:
polyphenols and flavonoids
alkaloids, such as caffeine and theobromine
minerals, such as fluoride and aluminum
When it comes to cancer treatment and prevention, it's the polyphenols that are important. Green tea has higher amounts of these chemicals than black tea because the fermentation process alters their composition. Of all the different polyphenols, one seems to stand out above the rest. EGCG (epigallocatechin gallate) is a very powerful antioxidant and is believed to be an important player in the therapeutic qualities of green tea. How it works is still unclear, but it may inhibit cell-replication enzymes (preventing cancer growth) as well as other cellular processes that are too complicated to get into.
In various studies, rats with breast tumors were given green tea to drink and compared to similar rats that only drank water. The green-tea-rats had reductions in tumor size, and new tumors were slower to develop. There don't seem to have been any studies on humans yet.
For humans to gain any health benefits, you would need to drink around 3-4 cups every day (without milk or sugar). For people who are at-risk for developing cancer, it might be a worthwhile avenue to pursue. I think for the everyday person, that's a lot of tea to drink. You can also get your green tea in capsule form, but there have been no studies done on the effectiveness of such pills.
A major concern with drinking so much tea is the caffeine. Though there is less caffeine in tea than in coffee, it does start to add up when drinking large volumes. But can you switch to decaf? The answer is, maybe. It all depends on how your chosen tea is decaffeinated. Tea that has been decaffeinated with a solvent (such as Ethyl Acetate) is going to have a much lower level of EGCG, than a tea that has been processed with a water/carbon dioxide method. Water decaffeinated tea will retain almost 95% of its EGCG.
So there is one more good reason for us women to drink green tea. I read that green tea might also help against prostate cancer, but that's another article altogether.