Is there a veterinary oncologist in your area who you could see? They would be able to give you a better idea of what your options are. My first instinct was well.. it's on her leg... dogs do pretty well with three legs! I can't imagine your only choice is euthanasia. If you don't have a veterinary oncologist within traveling distance, they get a second and third opinion from other vets in the area. I wouldn't just go with what the one vet said.