Osteosarcoma is a highly angiogenic cancer arising from bone. These tumors are thought to arise at the metaphysis from the medullary cavity of the bone.
Dogs with osteosarcoma most commonly exhibit lameness or pain at the metaphyseal sites, and radiographs often reveal bone lesions.
Large and giant breed dogs are more likely to be affected, possibly because of the rapid growth that occurs in young larger breed dogs.
Previous bony fractures are also thought to possibly cause osteosarcoma, because of chronic bone irritation sometimes associated with injury.
Osteosarcomas are most likely to be found on the distal radius and the proximal humerus of the forelimb.
A key indication of osteosarcoma is a bone lesion not crossing a joint space that has evidence of bone lysis.
Radiographs may reveal lung metastases, which occur only in late stages of this tumor. If they are present, the dog has a poor prognosis.
90% of dogs will die of metastatic disease, even though metastasis may not be initially evident on radiographs.
Metastasis to the lungs often becomes evident after surgical removal of the primary osteosarcoma.
High-detail radiographs of the lesions and radiographs of the thorax are the first step towards diagnosing osteosarcoma in dogs.
The diagnosis is confirmed through a biopsy, taken at the time of limb amputation. Multiple biopsies are often taken from the middle of the tumor to confirm diagnosis.
Treatment of Osteosarcoma in dogs
Amputation of the limb affected by the osteosarcoma is the most common treatment for this disease.
This aggressive treatment is often results in a reasonable quality of life for the dog and offers a good prognosis.
In some cases, the affected limb may be spared, especially in dogs with a preexisting orthopedic or neurological condition, where limb loss would prove severely debilitating.
In such situations, radiation therapy is often used to treat osteosarcoma, along with surgical excision of the tumor along with a large section of the bone.
Chemotherapy is recommended after surgery in order to attack remaining cancer cells that may not have been removed.
Though tumor excision may salvage the limb, there are potential complications associated with this option.
The two most common are:
1) recurrent local disease; and
2) allograft infection (since bone allografts are often necessary to replace some of the excised bone).
Even after limb amputation, osteosarcoma often recurs as metastases to the lungs. The reason for this recurrence is thought to be due to cancer cells which had already seeded the lungs, but which were unable to grow when the primary, or first, tumor was present on the limb.
Primary tumors are known to produce certain angiogenesis inhibitors, such as angiostatin and endostatin, which circulate in the bloodstream and act to suppress the growth of cancer cells in distant organs.
When the primary tumor is removed by amputation, the presence of these inhibitors is reduced, allowing those distant cancer cells to grow.
Dogs with osteosarcoma were recently shown to secrete angiostatin in their urine, which disappears once their tumor is removed.
Hence, antiangiogenic therapy may prove to be useful to treat osteosarcoma in the limb, as well as to prevent metastases.
In animal studies, various angiogenesis inhibitors have been shown to reduce osteosarcoma growth, including anti-VEGF antibody, AGM-1470, and vitamin D3. These agents have not yet been formally studied in formal canine trials.
