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Osteosarcoma (Bone Cancer)
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.
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. 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
Chemotherapy - A drug called "cisplatin" 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 cisplatin, 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.
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