[jlaack]

Yes the tumor can grow that quickly. Osteosarcoma is a very aggressive cancer. It concerned me a bit that your vet gave you a 3 week timeline on his life. While it can be that quick or quicker it can also be longer. On average dogs with OS live 3-6 months without any treatment. Did he xray the lungs as well? If the answer was yes and the disease is in the lungs than I would agree that you won't have much time left with Lucas.

OS is very characteristic and can be dx by xray alone. I wouldn't do the biopsy because it probably isn't warranted and it can actually cause the cancer to spread. If you are at all uncertain about the dx having a radiologist at Cornell view the xrays would be a great way to obtain confirmation.