What stage does osteosarcoma have to be at before bones are weakened enough to break easily?

I believe 50% of the: bone involvement is likely to result in a pathological fracture. U should see and ask your local Ortho-Oncologist about this, as he has seen and examined you and your scans/XRs etc, so he can answer you better,. rather any of us, who may not fully appreciate the extent of your lesion, without seeing all the documentations/scans. Good luck.