Topics in Oncology: Osteosarcoma - An Oncologist's Perspective
Sarah Sheafor, DVM, Dip ACVIM

Osteosarcoma is the most common primary bone tumor in dogs. It most commonly occurs in the long bones of middle-aged, large and giant breed dogs. The distal radius is the most frequent location for osteosarcoma. Dogs present with signs of lameness and swelling. A presumptive diagnosis is often made based on signalment, physical examination, and radiographic findings of a osteolytic, osteoproliferative or mixed bone lesion. Differential diagnoses for these lesions include other primary bone tumors (chondrosarcoma, fibrosarcoma, hemangiosarcoma), lymphoma or multiple myeloma of bone, tumors metastatic to bone (carcinomas), systemic mycoses, and bacterial osteomyelitis. Osteosarcoma can be definitively diagnosed via bone biopsy. Thoracic radiographs are an important part of the diagnostic workup, as 5%-10% of dogs will have visible metastatic lesions at the time of initial presentation. Routine screening of blood and urine are also essential to evaluate the overall health of the patient, and to determine the best therapeutic options.

Local treatment is the first step in therapy. Bone tumors are painful, and excision is the best way to relieve the pain. Amputation has long been the mainstay of therapy for osteosarcoma. Limb-sparing surgery for distal radial OSA is also successful. Palliative radiotherapy is an option for dogs who cannot have amputations (eg., those with disabling orthopedic or neurologic conditions). Palliative radiation consists of 3-4 treatments of megavoltage radiation. This therapy is effective at controlling pain in up to 70% of patients. The tumor is not affected, and the tumor-weakened bone may fracture as the patient uses the leg more normally. Regardless of the form of local therapy, if adjuvant therapy is not used, most patients succumb to metastases within 3-4 months. The one year survival of dogs treated with amputation alone is less than 10%.

Systemic therapy is effective in dogs with osteosarcoma, producing 40-60% in one year survival rates. We now have three standard treatment options for dogs with OSA, enabling us to pick the drug which best fits the patient's needs. The gold standard drug is cisplatin. Cisplatin is not a good choice for dogs with renal disease, or dogs with significant heart disease who cannot tolerate fluid diuresis. Cisplatin is given every 21 days as an intravenous infusion. Diuresis with saline is required both pre- and post-therapy meaning that displatin therapy requires a one day hospital stay. Carboplatin is the second-generation form of cisplatin. It is given as an IV bolus drug, so is appropriate for outpatient therapy. Carboplatin is less nephrotoxic than cisplatin, and doesn't require any diuresis. It is given every three weeks, as well. Four treatment cycles of cisplatin and carboplatin are ideal. Doxorubicin (Adriamycin) is also effective against osteosarcoma micrometastases. It is inappropriate for dogs with heart disease or those who may have occult cardiomyopathy (Dobermans). For osteosarcoma, it is given as a 30-40 minute intravenous infusion every two weeks for five treatments. Echocardiographic monitoring before the first, third and fifth treatments is recommended. All of these systemic therapies are available through the SouthPaws oncology service.

We are excited to be starting a project with a veterinary oncology researcher at the National Cancer Institute for dogs with osteosarcoma. Dogs that are proven by biopsy to have osteosarcoma will be eligible for inclusion into this new treatment protocol. They will be given carboplatin and either a placebo, or interleukin-12. After the first treatment, a standard amputation can be performed. Two more carboplatin +/- IL-12 treatments will follow. Interleukin-12 is a newly identified cytokine that activates the helper T cells. It has shown anti-tumor activity in rodent models, without the toxicity often associated with IL-2 therapy. The study will provide free carboplatin, and, of course, the IL-12 or placebo therapy. If you have a patient who may qualify for this study, I would be happy to help the owners decide whether this study, or one of the standard therapies is most appropriate for them and their dog.

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