Osteosarcoma (OSA) is the most common tumor of bone, accounting for over 80% of primary bone tumors. OSA occurs most frequently in the long bones of older, large-breed dogs. Any lameness in an older, large breed dog should prompt radiographic evaluation, especially if it responds inadequately to short-term rest and NSAID therapy. OSA must be distinguished from other primary and metastatic bone tumors. The behavior of OSA in cats is very different than OSA in dogs.
The behavior of OSA has been evaluated several times in the past 50 years through retrospective studies. It is known to metastasize soon after it is spawned in the marrow cavity and the most common site for metastasis is the lung. Only about 10% of dogs are discovered to have obvious metastasis at the time of initial diagnosis. However, regardless of how soon amputation is performed, the mortality rate is 90% in the first year for animals receiving amputation alone. Apparently, factors associated with the primary tumor prevent the micrometastases from growing. Knowledge of this behavior has resulted in the strategy of instituting chemotherapy soon after removal of the primary tumor. This strategy has resulted in the improved survival of nearly 50% of dogs for the first year and 20% for the second year following treatment. The metastatic rate in cats is much lower.
Treatment options for osteosarcoma include palliative radiation therapy, amputation alone, amputation followed by chemotherapy, and limb-sparing surgical techniques followed by chemotherapy. Radiation therapy is useful in providing pain relief, and prolonging the animal’s comfortable life. Survival time is usually 4-6 months with radiation therapy, unless the animal develops a pathologic fracture, necessitating amputation or euthanasia. Repair is usually unsuccessful in bone compromised by tumor.
Amputation alone can also prolong the animal’s comfortable life. Survival times are, again, 4-6 months. Amputation in the cat results in a 70% 1-year survival with surgery alone. Most dogs succumb to metastatic disease, inapparent at initial presentation. The addition of chemotherapy to amputation at least doubles survival times. Median survival is around 1 year, while 25% of animals will survive 2 years. There are three chemotherapy drug options available (carboplatin, cisplatin, and doxorubicin), varying with cost and potential side effects, but all affording essentially the same survival (see protocols). Chemotherapy in the face of gross tumor is generally unrewarding, so serial 3-view chest radiographs, usually prior to the initial treatment and then every other treatment to monitor for metastatic disease is advisable.
It is difficult for some owners to accept the option of amputation. However, for most large breed dogs with otherwise normal bone and muscle structure, amputation is not as limiting as it would seem. Most owners and pets acclimate to amputation within a few days to two weeks. Surveys of owners who wanted to avoid amputation, but ultimately accepted that option, have revealed that 90% felt the amputation resulted in fair, good or excellent quality of life.