Nancy (I'm Nancy too), I'm so sorry you're dealing with this. When Winnie was diagnosed, we did the biopsy---not being very knowledgeable about osteo (this was almost 5 years ago)---and I wish we hadn't. It came back not 100% conclusive, but given the odds, we did the amputation. After they took the leg off, my vets, on their own, for their knowledge, attempted to biopsy it again. The leg completely shattered at the tumor site. So in retrospect, I wouldn't have wasted that time waiting for biopsy results. I just thank God that we got that leg off before it shattered when my precious girl was 4 legged, and Winnie would have been in excruciating pain. And we would have had an emergency amputation, costing a lot more than it did.
This is such a hard time. But you have this going for you---you know in your hearts what you are going to do. Once making that decision, you are able to move forward and help your girl in any way you can. And her lungs are clear! That is important, and very positive news.
Please come here, and Circle of Grey, to talk, ask questions, vent. There are, unfortunately, too many of us who have dealt with the monster, and know what you're going through
Cindy is in my prayers.