Osteosarcoma

WHAT IS OSTEOSARCOMA?
Osteosarcoma is by far the most common bone tumor of the dog, usually striking the leg bones of larger breeds. Osteosarcoma usually arises in middle aged or elderly dogs but can arise in a dog of any age with larger breeds tending to develop tumors at younger ages.
Osteosarcoma can develop in any bone but the limbs account for 75-85% of affected bones. Osteosarcoma of the limbs is called "appendicular osteosarcoma."
Osteosarcoma develops deep within the bone and becomes progressively more painful as it grows outward and the bone is destroyed from the inside out. The lameness goes from intermittent to constant over 1-3 months. Obvious swelling becomes evident as the tumor grows and normal bone is replaced by tumorous bone.
Tumorous bone is not as strong as normal bone and can break with minor injury. This type of broken bone is called a “pathologic fracture” and may be the finding that confirms the diagnosis of bone tumor. Pathologic fractures will not heal and there is no point in putting on casts or attempting surgical stabilization.
HOW DO WE KNOW MY DOG REALLY HAS AN OSTEOSARCOMA?
RADIOGRAPHS: One of the first steps in evaluating a persistent lameness is radiography (x-rays). Bone tumors are tender so it is usually clear what part of the limb should be radiographed. The osteosarcoma is creates some characteristic findings:
The “lytic lesion” – looks like an area of bone has been eaten away.
The “sunburst” pattern – shows as a corona effect as the tumor grows outward and pushes the more normal outer bone up and away.
A pathologic fracture may be seen through the abnormal bone.
Osteosarcoma does not cross the joint space to affect other bones comprising the joint.
In most cases, radiography is all that is needed to make the diagnosis but sometimes there are ambiguities.
BIOPSY: A tiny section of bone can be removed for laboratory analysis. This type of analysis is considered to be absolute proof of diagnosis. The procedure is associated with some pain and our local oncologists have suggested that biopsy is not needed if the radiographs show an obvious bone tumor. If there is any question about the lesion on the radiographs, a bone biopsy should provide clear results.
Sometimes a bone tumor is surrounded by an area of bone inflammation and it may be difficult to get a diagnostic sample and several samples must be taken. These samples are too small to cause a pathologic fracture.
Amputation of the affected bone is recommended for any tumor involving bone. When the malignant structure has been removed, it is submitted for biopsy and the diagnosis confirmed at that time. Biopsy before amputation is felt to simply add a painful procedure to the patient and, if possible, is reserved for tissue already amputated.