Bone tumors in dogs are very similar
to those in humans. The small region between the shaft and ends of
the long bones (the metaphysis, where growth occurs) is the most
common site. These tumors are usually high-grade, aggressive, and
usually spread to other parts of the body. The lung is most
commonly involved.
Osteosarcoma tends to affect larger
breeds with a slight increase in incidence with age. Males are more
likely to be affected than are females.
Weight-bearing long bones of the
legs are most frequently involved, especially the metaphysis of the
radius. Breeds which weigh over 80 pounds are more likely to
develop bone cancer than dogs weighing less than this amount. Also,
the rates of developing bone cancer between breeds increases with
standard height of the breed independent of the dogs' weight. This
means, for example, that when you consider two breeds which weigh
over 80 pounds, say an Great Dane and a Saint Bernard, the Great
Dane has a higher likelihood of getting osteosarcoma because he is
taller at the withers. However, within a given breed, heavier
animals are more likely to develop the disease.
As in human children, development of
bone cancer in dogs is related to rapid bone growth. It is
postulated that strenuous activity causing microscopic fractures of
bones during periods of rapid growth induces cancer formation.
Since taller dogs have a longer growth period than smaller ones,
they are exposed to the risk of getting the cancer for a longer
period of time. Likewise, heavier dogs are more likely to stress
their developing bones leading to the microscopic fractures that
start the tumor development process.
Ionizing radiation (as is given in
radiation therapy) and having a metallic implant in the repair of a
fracture are both associated with developing osteosarcoma. However,
given their rarity in dogs, neither of these two factors is likely
responsible for a significant number of bone tumors.
Diagnosis
Radiography - A radiograph (x-ray)
of the bone suspected to be abnormal is necessary. Most primary
bone tumors have a very characteristic appearance on radiographs.
The bone appears to have been "eaten away" by the tumor. Sometimes,
a tumor will not have the characteristic radiographic appearance.
In any case, if a tumor is suspected based on the x-ray, a bone
biopsy should be performed. Bone Biopsy - In most cases, it is
preferable to have the patient under a brief general anesthetic.
This is for the patients comfort. In cases where anesthetic is
considered a risk, the procedure can be performed with a local
anesthetic, but may be associated with some discomfort. A small
instrument called a "Jamshidi" biopsy needle is used to obtain a
"core" of bone. A tiny incision is made through the skin, and the
needle placed to obtain the sample. In most cases, the incision is
so small that sutures are not necessary. The sample taken is then
sent to a pathology laboratory for analysis.
Other Tests - In
patients diagnosed with osteosarcoma, it is extremely important to
determine if the cancer has spread. Osteosarcoma can go to any
tissue of the body, but is most commonly found in lungs or in other
bones. Up to 10% of patients will have visible cancer in the lungs
at the time of the initial diagnosis.
A complete work-up involves the
following tests: Complete blood count- Serum chemistry profile
& urinalysis - Thoracic (lung) radiographs - Radiographic bone
survey of appendicular skeleton - Bone biopsy ยท
It is not always necessary to do all
of the tests listed above; however, these help give us an
indication of the stage of your pets cancer and his/her general
health and ability to undergo treatment
Treatment
The primary tumor is usually removed by amputation
of the affected limb. Through innovative limb-salvage procedures,
some patients can save their limbs and still be successfully
treated. Once the primary tumor is removed surgically, chemotherapy
is given. We know that most of the tumors have spread
microscopically at the time of diagnosis. We won't see this on
radiographs, but we know it's there. If an amputation is NOT
followed by chemotherapy, patients will not live any longer than if
no treatment at all were given. The reason for this is that the
microscopic disease already present in the lungs grows. The purpose
of chemotherapy is to prevent or delay this growth. Chemotherapy -
A drug called "cis-platin" is our first choice in treating
osteosarcoma. The first dosage is given immediately after surgery
(in fact, the drug treatment is started while the patient is in
recovery from surgery). Patients are given a total of 4 dosages at
21 day intervals. The treatment takes from 4-8 hours to administer.
In order to protect the patient's kidneys from possible toxic side
effects of cis-platin, high volumes of fluid are given immediately
before and after administration. The most common side effect
associated with treatment is nausea or vomiting. This can usually
be adequately controlled with anti-emetics and subsides by the time
the patient is discharged. Side effects at home following treatment
are uncommon. While hair loss and severe illness is associated with
chemotherapy in humans, it is rarely seen in animals
