So sorry to hear that your furbaby has cancer. My first Golden, Natasha, also develped breast cancer at the age of 9 1/2. She had surgery and the tumor was back six weeks later. After that she had several chemo treatments but they didn't help. Finally, when the vet told me that it could spread to the lungs and that there wasn't anything else he could do, I had to come to the very difficult decision of letting her go to the other side of Rainbow Bridge.
This was in 1986, veterinary medicine has advanced a lot since then. Perhaps you could check some Web sites to find out what's new. Here are some addresses:
www.vetinfo.com/
www.petvets.com/
www.merckvetmanual.com/mvm/index
I hope you find a way to help your pup. Best wishes to find a cure, my prayers are with you.