Well I could be wrong here, and others may feel free to correct me if I am, but from what I understand the risk of mammary gland cancer does not come from the spaying itself, but rather from the fact that your dog has been in heat. One of the main reasons for spaying prior to a female going into heat is that the mammary glands don't develop if they don't go into heat, hence no risk of mammary cancer. Since your girl has been in heat she has developed mammary glands and her risk of cancer is greater. It is not directly related to the surgery. Again, this is just based on what my vet has said to me and how I interpreted it. Either way, she'll be happier without the heat cycles.