Canine OSA occurs most often in dogs 7-9 years of age, but it has been reported in young (1-2 year old) dogs as well. Large breed dogs are at a much higher risk of developing OSAs (up to 150 times greater risk). A slight predilection for males has been demonstrated, but neutered animals of either sex are twice as likely to develop OSAs as intact animals.
Osteosarcomas can develop in many sites both skeletal and extraskeletal; however, extraskeletal OSAs only comprise a very small percentage of all canine osteosarcomas. Of the skeletal OSAs, most (approximately 75%) are appendicular. (Therefore, the rest of this article will deal specifically with OSA of this location.) The most common locations for these appendicular osteosarcomas are the distal radius or proximal humerus followed by the proximal and distal femur and tibia. The metaphyseal region is affected most often and the tumor rarely crosses the joint.
Highly aggressive and metastatic in nature, over 90% of all clinically significant OSAs have already micrometastasized by the time of diagnosis. Most metastasis happens via menatogenous spread to the lungs and other bones, but lymph node metastases have been reported.
The typical presentation of canine OSA is lameness of the affected limb with or without a noticeable swelling or mass at the tumor site. The lameness is either due to periosteal inflammation, microfractures, or pathologic fractures. If swelling is present, it is likely due to extension of the tumor into the surrounding soft tissues.
Diagnosis of canine OSA is made based on the clinical presentation described above, a complete orthopedic and neurologic examination (to rule out other causes of lameness), physical examination, regional radiographs and bone biopsy. Radiographic signs include loss of cortical bone, periosteal proliferation, palisading cortical bone, Codman's triangle, loss of fine trabecular pattern in metaphyseal bone, and metaphyseal collapse with a pathologic fracture. Although these changes are distinct, they are not pathognomonic for OSA. Other conditions such as fibrosarcoma (FSA), chondrosarcoma (CSA), and fungal osteomyelitis may be indistinguishable radiographically. This is why a bone biopsy is needed to help obtain an accurate diagnosis.
Bone biopsies of OSAs can be performed with open or closed techniques. Whichever technique is used, the sample should only include one cortex to maintain as much strength as possible at the biopsy site. Multiple samples should be taken from both the center and the margins of the lesions for a more accurate diagnosis.
Histologically, OSAs consist of malignant mesenchymal cells that appear enlarged and polygonal to spinyloid in shape which produce an osteoid matrix. This osteoid matrix is a distinguishing feature of OSAs. Nonosteogenic bone tumors (FSA, CSA) do not produce this matrix. Subclasses of osteosarcomas are determined based on the characterization of the cells as well as the type and amount of matrix present. Subclasses include osteoblastic, chondroblastic, fibroblastic, poorly differentiated, and telangiectatic osteosarcomas; however, there is no evidence of different biological behavior between the subclasses.
Once a diagnosis has been made, a full evaluation for metastases must be done. Right and left lateral thoracic views must both be done in addition to the ventrodorsal so that all lung fields can be accurately evaluated. Only 10-15% of cases will have detectable metastases at the time of diagnosis in spite of the 90% micrometastatic rate by the same time. To evaluate for bone involvement elsewhere, nuclear scintigraphy or full skeleton radiographs can be performed.
Treatment of canine OSA has one of two goals: palliation of pain and lameness or curative intent. Palliation of the pain is done in cases where metastasis is already evident or to comply with the owner's wishes. It includes the use of analgesics, radiation therapy, limb amputation, and metronomic chemotherapy. Curative intent options include combining different modes of therapy such as surgery, radiation therapy, and chemotherapy. However, no matter the combination used, 80% of dogs with OSA die due to the high metastatic potential of the tumor.
Curative intent surgeries have traditionally involved limb amputation, but recently, more limb sparing procedures are being done with comparable survival times. In the limb sparing techniques, the tumor is removed with marginal soft tissue resection. A cortical allograft is then placed in the bone void; the bone is then plated in place. Complication rates for this procedure can exceed 50%, but for dogs with other orthopedic or neurologic function, it may be the best option. New techniques are being explored to decrease the rate of complications associated with the cortical allografts.
Since metastasis is the most common cause of death in dogs with osteosarcoma, the addition of chemotherapy into curative intent protocols is vital to longer term survival. They are used either in combination with surgery or radiation therapy in an attempt to decrease the metastatic rate. Cisplatin, Carboplatin and Doxorubicin are examples of agents currently used to help control metastatic disease. Their use markedly increases survival times beyond those of amputation or tumor removal alone.
Prognosis for patients with OSA is based on many factors. Poor prognostic indicators are animals under 7 years of age, large tumor volume, tumor location in the proximal humerus, elevated alkaline phosphatase (total and bone specific), failure of these levels to return to normal by 40 days post-operative, high tumor grade, and presence of metastasis. Survival times after treatments vary greatly and much research is currently being done in this area. Recently, a median survival time of 7 months was reported for dogs receiving radiation therapy along with chemotherapy; whereas a combination of surgery and chemotherapy showed more encouraging median survival rates of 235-366 days with up to 28% surviving two years after diagnosis.
-by Lori Fruit, Class of 2005
-edited by Dr. Leon Thacker, ADDL Director
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