Another bad one.
Osteosarcoma (OSA) is bone cancer. It affects an estimated 8,000-10,000 dogs in the US each year and is more common in dogs than in any other species including humans. OSA is most common in large to giant breed dogs and the most common location is the distal radius (forearm). 75-80% of OSA lesions are in the long bones of the limbs versus the other bones of the body except in small breed dogs where they occur in equal frequencies on the limbs vs other regions. OSA of the mandible or distal to the carpus (wrist) or tarsus (ankle) may carry a slightly better prognosis.
The typical presentation is a large breed middle aged to older dog (although it can occur in younger dogs) with a sudden onset of lameness and/or a swelling in the limb that is not associated with a joint. Presumptive diagnosis is frequently possible with radiographs alone. Biopsy can used to confirm the diagnosis but is often only performed it it will change the treatment plan as it increases the risk of a secondary fracture.
Amputation plus chemo (various protocols) remains the gold standard of treatment. Average survival times with the various protocols vary between 250-500 days with one year survival rates only reaching 50% at best.
Limb sparing surgeries are available for dogs who are not candidates for amputation and have similar success rates when combined with chemotherapy. Radiation can also be used to provide pain control on dogs who are not pursuing amputation. There has been a 70% response rate in these dogs and it generally lasts for 2-3 months. Typically dogs get 3-4 treatments, not the more aggressive 3-4 week course discussed before.
Additional chemo, radiation, and nuclear medicine therapies/protocols as well as other novel treatments are being investigated to try and give us better options for this devastating disease.
OSA is rare in cats but carries a much better prognosis as it rarely metastasizes. Many cases are cures by amputation/surgery alone.