If this were my dog, I'd get the leg amputated and then start chemotherapy using Pamidronate (a bisphosphonate). This has been shown to achieve survival times of ~ 12-18 months with osteosarcoma
. 1 year in a 5 yr old dog is ~ 20% longer lifespan so it's definitely worth it. They will usually have a normal quality of life during this time and we consider euthanasia when the lung masses start to form and grow causing coughing and respiratory signs.
If you don't want to go this route, then I would at least amputate the limb as that is the only way to remove that source of pain and avoid a fracture at the tumor site.
If you still don't want to amputate, then I would have her on a combination of meds:
1. A Bisphosphonate chemo drug to prevent further breakdown of bone
2. NSAIDS- anti-inflammatories like Rimadyl
3. Tramadol- an opiate type pain med
4. Gabapentin or Amantadine- good for chronic pain if needed
I hope this helps, let me know if you have any other questions.