Current treatment options for dogs with appendicular osteosarcoma
Osteosarcoma is estimated to occur in more than 8,000 dogs a year in the United States.1 Although amputation has traditionally been used to palliatively manage affected dogs, treatment modalities including limb salvage, radiation therapy, and chemotherapy are evolving. This overview of emerging therapies will help you educate owners about treatment options for dogs with appendicular osteosarcoma.
CANINE OSTEOSARCOMA: AN OVERVIEW
Osteosarcoma is the most common primary bone neoplasia affecting dogs and accounts for nearly 6% of all canine malignancies.2 Large and giant breeds are more commonly affected, and males are more often affected than females by a ratio of 1.5:1.3-6 Although osteosarcoma has been reported in dogs as young as 6 months of age,7 there is a bimodal age distribution with the initial peak at 3 years and the second peak at 8 years.8 If left untreated, most dogs will be euthanized within three months of diagnosis.5,8 Osteosarcomas are classified as simple (bone formed in collagenous matrix), compound (both bone and cartilage are present), or pleomorphic (anaplastic cells predominate, with only small islands of osteoid present).9 Classification has also been based on cell type and activity (osteoblastic, chondroblastic, or fibroblastic), radiographic appearance (lytic, sclerotic, or mixed), or origin (central, juxtacortical, or periosteal).9Clinically, osteosarcoma is characterized by aggressive local bone destruction with invasion into the surrounding soft tissues.9 Seventy-five percent of tumors arise in the metaphyseal region of the long bones.5,8,9 The remaining 25% occur in the axial skeleton or soft tissues.5,8,10 The distal radius is the most common site in dogs.5,8,9 Other less common appendicular sites include the proximal humerus, distal ulna, proximal or distal femur, and proximal or distal tibia. Osteosarcomas are typically rapidly growing, painful tumors.9
Grossly, central or interosseous osteosarcomas have a gray-white appearance and contain variable amounts of mineralized bone.9 Rapidly growing intramedullary osteosarcomas often have large pale areas of infarction and irregular areas of hemorrhage.9 Neoplastic tissue tends to fill the medullary cavity of the metaphysis and can extend proximally and distally. Typically, however, this extension does not penetrate growth plates or articular cartilage to enter the joint space.9 Cortical bone is destroyed with varying amounts of new reactive bone surrounding the area, while neoplastic cells penetrate and undermine the periosteum.9 New bone formation may be abundant and widespread or minimal.9
Radiographic evidence of metastasis is often not present at the time of initial diagnosis.1 Common sites of metastasis are the lungs and other skeletal tissues. Metastasis has also been reported in the visceral organs, brain, and subcutaneous tissues.1 Although less than 10% of affected dogs have radiographically detectable metastasis at the time of presentation, 90% of dogs die of metastatic disease within one year of diagnosis if not treated with chemotherapy and surgery.5,8 This suggests that micrometastatic disease is usually present in the lungs at the time of diagnosis, possibly in a dormant state.5,8 Depending on the location and stage of the disease, surgery, radiation therapy, and chemotherapy can be used in a variety of treatment protocols.
Amputation is the most common surgical treatment for appendicular osteosarcoma and is recommended for most dogs with osteosarcoma. Limb amputation provides pain relief, relatively short anesthesia times, a low risk of surgical and postoperative complications, a short convalescence, lower costs than limb-sparing surgery, and a limited risk of tissue contamination or local recurrence because of incomplete resection.11 Some owners, however, are reluctant to pursue amputation.12,13 This reluctance is unfortunate since most dogs, including the larger breeds, function well after amputation, and owners who elect amputation are typically satisfied with their pets' quality of life after the procedure.11,14 Body size, age, and forelimb vs. hindlimb do not appear to influence the rate of recovery, owner satisfaction, or the ability of a dog to adapt after amputation.12 Poor candidates for amputation include dogs with advanced pulmonary metastasis, marked or severe orthopedic or neurologic problems in other limbs, and severe obesity.15