Hi and welcome to you and Archie. I'm so sorry for the dual diagnosis! She sounds like such a sweet girl and has been through so much. And let me just say that you two are incredibly kind humans for adopting her at an older age.

So, it sounds like the vets think that despite her extra weight she will do fine on three legs. That's great! Dogs can lose weight pretty quickly with our help, and recovery is a good time to get things moving along since pain medications do affect appetite in most dogs. The best vets tell us that neither size nor age should impact a dog's candidacy as a Tripawd. Sounds like she's a good candidate overall.

What I'm most concerned about is the two cancers. Have you seen an oncologist to get the mammary tumors graded? It would be good to know how far advanced that cancer is before proceeding.

The thing about osteosarcoma is that amputation is the fastest way to help get rid of the awful pain it causes. Bone pain is the most severe there is, for humans or dogs. But you also want to make sure that the mammary cancer isn't so advanced that she won't have time to recuperate from the surgery. For older dogs generally it takes a few weeks longer than younger ones, and add a little more time for overweight dogs to get their bearings.

There are other palliative treatments for osteosarcoma that wouldn't require amputation, such as cementoplasty or stereotactic radiation therapy. Did your vet mention any of those options to you?  These newer therapies can bring down pain levels and buy her more quality of life if they are an option for you. Cementoplasty tends to cost about the same as amputation surgery but SRT is quite expensive without pet insurance.