Canine Osteosarcoma (OSA)
OSA is the most common primary bone tumor of dogs and is estimated to account for roughly 85% of tumors in the canine skeleton. It is most often a tumor of the front limb with tumors near the "wrist" (in the radius or ulna) common and tumor in other sites of the front and rear leg somewhat less common. Another rule of thumb often stated is that these tumor occur away from the elbow and towards the knee. OSA can, however, occur in any bone. Middle aged to older large to giant breed dogs are most often affected by the form of tumor involving the long bones such as the radius, ulna, humerus, femur and tibia. The cause of this tumor is largely unknown.
What you might see/ Clinical presentation
The most common sign seen in dogs with OSA is lameness. Lameness may develop suddenly (for example, after vigorous activity), or may develop more slowly. Depending on the location of the tumor, a swelling or mass-like effect could be seen in the affected leg. Any lameness in a large-breed dog that does not promptly resolve with symptomatic therapy should be further investigated (examination, radiographs).
Biological behavior of Osteosarcoma
OSA is both a locally invasive tumor and a tumor with a high likelihood of spread (metastasis) to other organs, most commonly the lung, although metastasis to other organs is possible. Most (approximately 90-95%) of dogs with OSA are considered to have metastasis at the time of their diagnosis, although metastasis will only be evident at the time of diagnosis in approximately 10% of dogs and in others the metastatic tumors are considered to be microscopic. Tumors in bone are quite painful to affected dogs, accounting for the lameness that is seen as the most common presenting complaint in affected dogs.
Clinical staging (determination of the extent of the tumor)
Radiographs of the affected leg, and radiographs of the thorax are the most critical elements of clinical staging; dogs with enlarged lymph nodes will also typically have the lymph nodes sampled for microscopic examination. Radiographs of affected legs are often highly suggestive of the diagnosis, and biopsies are not necessary for all dogs with compatible lesions on radiographs. Often, obtaining blood for a complete blood count and biochemical profile, and a urinalysis will be advised as these can help assess overall health and provide information that potentially influences treatment recommendations. When aggressive therapy is being considered, a CT examination of the lungs will also be recommended to screen the lungs for metastasis even more precisely than the radiographs can show us.
Amputation to remove the primary tumor, combined with chemotherapy to treat the metastatic disease we can’t yet see, is the best treatment for preservation of quality (relief of pain) and quantity of life for dogs with the limb form of OSA. Not every dog is a candidate for amputation and the decision to amputate is often difficult for owners. Dogs with severe arthritis in the unaffected limbs, or weakness due to neurologic disease may not be candidates for amputation but most other dogs are. Dogs can function on three legs much better than most owners think. Some institutions offer limb-sparing surgery, in conjunction with chemotherapy, but this can only be done when the tumor is in the radius or ulna near the carpal joint (wrist). Not every owner is prepared for the expense and time commitment to chemotherapy either. Generally therapy is done once every 3-4 weeks for 5 doses of chemotherapy. Drugs commonly used include cisplaitn, carboplatin and doxorubicin. Amputation without chemotherapy can remove the source of pain, but generally metastases will become apparent in 3-4 months.
Palliative therapies are those aimed at keeping the leg more comfortable so that the dog can live with it longer. There is no benefit expected in most dogs from the administration of chemotherapy drugs if the primary tumor is not removed. Radiation therapy will improve comfort in approximately 70-80% of treated dogs. Generally only a few large doses of radiation are administered. Administration of analgesics, which can be done in conjunction with palliative radiation therapy, can improve the level of comfort in some dogs. Additionally a class of drugs known as bis-phosphonates is being tried for dogs with osteosarcomas. These drugs inhibit bone resorption, or break down. They have been use in the human field for osteoporosis but also in the treatment of tumors which have metastasized to bone to relieve bone pain. Clinical trials are underway to determine if these drugs actually relieve pain in primary bone tumors of dogs.
Dogs with OSA in other bone locations are often treated with a combination of surgery, radiation therapy and chemotherapy.
The prognosis for dogs undergoing surgery to remove the primary limb tumor (ampuation or limb-sparing surgery) and treated with chemotherapy is fair to good. About half of dogs with OSA treated in this manner will be alive 1 year after diagnosis; approximately 25% of dogs treated in this fashion are alive 2 years after diagnosis, and occasional dogs will be cured. Dogs treated with either amputation or palliative radiation therapy live, on average, approximately 6 months before complication of the local tumor or metastatic disease cause death or prompt euthanasia.
Future treatment options
Many therapies being developed in the human field aimed at stopping metastatic disease could be helpful in dogs with osteosarcomas. Bisphosphonate drugs may also prove useful for palliation of tumor pain, particularly when combined with palliative radiation therapy. A Gene therapy trial administering a gene coding for a potent immune stimulant (IL-2) has shown promise in preliminary studies.
Key points Most dogs do amazingly well with three legs; amputation, however, is not appropriate for all dogs, particularly those with other orthopedic or neurologic diseases. For such patients, palliative radiation therapy is considered an excellent treatment alternative with a high likelihood of improving patient comfort. Unfortunately, most dogs will die from complications of OSA. Statistics, while useful, can never predict how an individual dog will fare with or without specific treatment.