What is Osteosarcoma?
Osteosarcoma is by far the most common bone tumor of dogs, usually striking the leg bones of larger breeds. Osteosarcoma usually occurs in middle aged or elderly dogs but can occur in a dog of any age; larger breeds tend to develop tumors at younger ages.
- Osteosarcoma can develop in any bone but the limbs account for 75-85 percent of affected bones. Osteosarcoma of the limbs is called appendicular osteosarcoma. It 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 to 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?
The lytic lesion looks like an area of bone has been eaten away. 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 creates some characteristic findings.
- 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 in the joint.
Radiography is almost completely diagnostic in most cases, but there are a few other far less common conditions that can mimic the appearance of a bone tumor, so a confirming test is going to be needed if one is to be complete. If a basic blood panel and urinalysis haven't been done, this would also be a good time to do so as basic information about liver and kidney function will be needed for treatment regardless of whether this turns out to be a bone tumor or not; plus, a tissue sample from the bone is needed for confirmation (see later).
Biopsy and Needle Aspirate Radiographs are close to being confirmatory but still they are not definitive. Since life and death decisions are going to be made, it is best to obtain a tissue sample for confirmation. This can be done by either biopsy or by needle aspirate.
A small piece of bone can be harvested surgically. The bone is preserved, sectioned, and examined under the microscope to confirm the diagnosis of osteosarcoma. There are several problems associated with this diagnostic. Sometimes a bone tumor is surrounded by an area of bone inflammation and it may be difficult to get a representative sample. The tiny hole that results when a core of bone is removed can create a weak spot and the bone can actually break. Even if the procedure goes well, often there is increased pain and lameness for the patient afterwards. With so many potential problems, most specialists have switched to needle aspirate for diagnosis.
With needle aspirate, a large bore needle is inserted into the area of the tumor and cells are withdrawn for analysis. A full core of bone is not removed, just a sampling of cells. This is usually sufficient to confirm osteosarcoma. If there is ambiguity, certain stains can often settle any questions the pathologist may have. With a Diagnosis Confirmed, Staging is the next Consideration early osteo Brooks An osteosarcoma patient's chest radiograph early in the disease. That said, how well treatments can be expected to work depend on whether or not the tumor spread has progressed so as to be visible. Because osteosarcoma spreads to the lungs as one of its first stops, chest radiographs are important in checking for visible tumor spread. If there is already visible tumor spread at diagnosis, this changes what treatments are recommended. Some specialists recommend nuclear imaging of the skeleton to identify any spread to other bones, which might also alter recommendations; however, this form of imaging is not readily available.