I really want to encourage you to do some research on the benefits of breastfeeding, not just for your baby but for you as well!! There is no better decision you can make. The tumors shouldn't effect anything but you should discuss that with your doctor. Breastfeeding actually greatly decreases your risk of getting breast cancer! Please do some searching online and you can also go into the breastfeeding community here and get some really great info.
I may be just using formula this time around when I do finally get pregnant and have another baby, but that depends. I never produced more than an ounce at a time total from both breasts, even with medication...so I'm going to try, but if I can't produce enough I may just nurse for a while anyway, it's better for baby.