I had a 13 yr old dog that had a mass on her bone. She would have needed a biopsy (not from needle as Growler mentioned but from surgery , she would have needed to be opened up) even if the vets thought it was 90% chances it was cancer. I didn't go through with it. I think a 13 yr old dog should just enjoy time they have left , and the owner should monitor very close to see if the dog is suffering , so they won't prolong the dog's life.
I know this isn't much advice for you :sad: I would wait for the results and ask for your vet's opinion. Good luck.