Introduction

Osteosarcoma (OS) is the most common bone tumour in dogs and is characterized by a highly invasive and metastatic behaviour. This tumour frequently affects middle-aged, large breed dogs (less than 5% occur in breeds smaller than 12 kg) and arises in 75% of cases in the metaphyseal area of bones of the appendicular skeleton. Males are more commonly affected than females. A cause for OSA is unknown although many etiologies have been stipulated (radiation, microtrauma, genetics, implants, nutrition).

Pathology

OSA is a malignant spindle-cell tumour characterized by direct formation of bone or osteoid tissue by tumour cells. OSA is an aggressive tumour with a locally invasive behaviour and a high rate of metastasis Common locations for OSA include the distal radius, proximal humerus, distal femur, proximal tibia, distal tibia, proximal femur. Multicentric OSA is rare (< 10% of cases). Pulmonary metastases are present in more than 90% of the patients at time of initial diagnosis. The prognosis for dogs with OSA without therapy is poor, less than 5% will survive longer than one year after diagnosis.

History, Clinical Signs, and Differential Diagnosis

Figure 1. Osteosarcoma in distal radio.

Dogs with OSA are often presented with an acute or chronic lameness and a visible swelling at the affected site. Muscle atrophy, a history of progressively decreased weight bearing, and pathologic fractures may be present. Differential diagnoses include other primary bone tumours (fibrosarcoma, chondrosarcoma, etc.), bone cysts and bacterial or fungal osteomyelitis in some endemic regions.

Diagnosis

The diagnosis of a bone tumour is easily obtained by regional radiography. Macroscopic pulmonary metastases (> 5 mm) are evident in 10% of cases at initial radiographic examination. The definite diagnosis is obtained by bone biopsy and histologic examination, but the procedure is not indicated in patients which strives limb sparing treatment.

Scintigraphy may be used to diagnose metastatic OSA. CT- and MRI-scans can be performed in some cases to estimate the extent of bony and surrounding soft tissue involvement.

Treatments Option

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Amputation

Surgical treatment of osteosarcoma by amputation is palliative and increases relief, thereby delaying euthanasia. Amputation usually eliminates the primary tumor with little to no reduction in mobility and quality of life for the dog. In two studies in Europe, dogs learned to walk well on three legs within a month. In one study survival of 65 dogs treated with amputation was 126 days; only 10.7% of dogs were euthanised after surgery. Surgery of any type is only palliative, and dogs with appendicular should be given chemotherapy.

Limb-Sparing Surgery

This procedure may be appropriate for dogs that are poor candidates for amputation (with other orthopedic or neurologic problems) or for dogs whose owners refuse amputation. During limb-sparing surgery, a cortical bone graft is used to replace the excised tumor, and arthrodesis of the nearby joint is usually performed. The best results are obtained by radial lesions or lesions of the ulna. It is possible to perform limb salvage for proximoscapular lesions, but function is poor, and the rate of postoperative complications is high.

Limb salvage is not an option for large lesions that involve more than 50% of the tumor that invade adjacent soft tissue. Some disadvantage of limb-sparing procedures is allografts from normal donors (usually dogs euthanized for another disease). Paste tumor has been used as an autograft for dogs with distal radial osteosarcoma. Low infection rates were similar to those from the use of an allograft.

Some surgeons use surgical metallic 'spacers' attached to the surgical scalpel space where the tumor is excised.