OSA arises in primitive bone cells, which produce osteoid (the organic matrix of bone before it becomes calcified). The production of osteoid differentiates osteosarcoma from the other sarcomas of bone such as fibrosarcoma and chondrosarcoma. OSA can cause cell destruction (lysis) and inappropriate production of bone, sometimes simultaneously.
The exact causes of OSA are unknown, but mutations in the p53, RB and PTEN genes, dysregulation of proto-oncogenes, and, rarely, metallic implants for fracture repair have all been implicated as contributors to OSA development. OSA metastasizes early in the course of the disease, with the lungs being the most common site for metastatic disease. Metastasis often is undetectable when the primary bone tumor is diagnosed, but 90 percent of dogs will die with metastatic disease within one year if limb amputation is the only treatment.4
Dogs with OSA of the limbs present with lameness and/or a swelling at the tumor site. Symptoms of axial OSA can vary depending on the affected bone. Radiographs will show varying degrees of lysis (bone destruction), osteogenesis (new bone formation), and soft tissue swelling (fig. 2). The classic radiographic pattern resembles a starburst of new bone radiating perpendicular to the affected bone. Unlike bacterial or fungal infections, which can affect both bones of a particular joint, OSA does not cross a joint.
Aspiration cytology can support the diagnosis of OSA, especially when the cells are stained to evaluate for the presence of alkaline phosphatase.5 ALP is a protein primarily found in the liver and bone. If a tumor is ALP negative, it is not OSA; however, positive staining for ALP is not specific for OSA, since tumors other than OSA can be ALP positive. A bone biopsy is needed for a definitive diagnosis, but this is an invasive procedure requiring general anesthesia.
Once a diagnosis of osteosarcoma is made, the extent of disease is determined (staging). Staging of OSA involves thorough palpation of all long bones, accessible axial bones and lymph nodes, three-view thoracic radiographs, and a minimum database of CBC/chemistry/urinalysis. Historically, survey radiographs of the entire body and/or bone scans (nuclear scintigraphy) were performed, but as access improves to newer imaging techniques, staging protocols will likely change.
Unfortunately, the current prognosis for OSA is poor. Even with removal of the primary tumor, pulmonary metastasis usually limits survival.
In future editions of this eUpdate, we will review the significant research that is ongoing to manage both the primary tumor and its spread. Morris Animal Foundation recently launched the Osteosarcoma Initiative to find new and innovative ways to improve survival and quality of life for dogs with OSA. Learn more about our Osteosarcoma Initiative, and all our canine health studies as we work together to give our dogs healthier, happier lives.
Fig. 2 OSA of the distal radius of a dog
1. Brodey, RS; and Riser, WH. Canine Osteosarcoma: a Clinicopathologic Study of 194 Cases. Clinical Orthop Relat Res. 1969; 62:54-64.
2. Cooley, DM; Beranek, BC; and Schlittler, DL., et al. Endogenous Gonadal Hormone Exposure and Bone Sarcoma Risk. Cancer Epidemiology, Biomarkers & Prevention. 2002; 11: 1434–1440.
3. Zwida, K; and Kutzler, MA. Non-Reproductive Long-Term Health Complications of Gonad Removal in Dogs as Well as Possible Causal Relationships with Post-Gonadectomy Elevated Luteinizing Hormone (LH) Concentrations. Journal of Etiology and Animal Health. 2016; 1: 1-11.
4. Withrow, SJ; Vail, DM; and Page, RL. Withrow and MacEwen’s Small Animal Clinical Oncology. St Louis (MO): Elsevier Saunders; 2013.
5. Barger, A; Graca, R; and Bailey, K et al. Use of Alkaline Phosphatase Staining to Differentiate Canine Osteosarcoma from Other Vimentin-positive Tumors. Vet Pathol. 2005; 42:161–165