Osteosarcoma is a tumor of the bone and is the most common primary bone tumor in the dog. It typically affects middle-aged to older dogs with a median age of seven years. However, it has been seen in dogs as young as six months and there is a small peak in age incidence at 18-24 months. It occurs more frequently in larger breed dogs. More than 75% of osteosarcomas develop in the long bones of the skeleton. The etiology of osteosarcoma is unknown. There has been speculation about a viral cause, but this has not been proven. There is also the possibility of a genetic cause involving the p53 gene and an Rb gene which normally act as tumor suppresser genes and for some reason are altered preventing their usual functioning. This too remains to be proven. Previous trauma has also been implicated.
Lameness is a common presenting symptom due to involvement of the long bones of the legs. There may be swelling to the affected leg and the dog may appear to be in pain. There may also be a history of mild trauma just prior to the onset of lameness that causes one to suspect a more benign orthopedic injury. If osteosarcoma is present in the scapula or shoulder of the dog, "dipping" shoulder movement may be exhibited with slight favoring of the leg on that side. There may be a history of intermittent lameness that becomes progressively worse. If the ribs are affected, a palpable, painful mass may be present. Osteosarcomas tend to metastasize to the lungs, but respiratory distress may not develop for months. Decreased appetite and decreased desire for physical activity are additional signs.
Physical exam and history. Radiographs to include at least two views of the suspected site. If osteosarcoma is suspected you may choose to have a Board-certified radiologist or orthopedic specialist evaluate the x-rays. Fine needle aspiration or surgical biopsy may also be used to diagnose osteosarcoma. Chest x-ray and or bone scans may be in order to determine if metastasis has occurred.
Successful treatment includes control of the primary tumor as well as prevention of metastasis. The standard treatment for osteosarcoma of a leg bone is amputation of that leg. Even large and giant breeds can function well after limb amputation and most owners are pleased with their dog's mobility and quality of life after this surgery. In addition to surgery, chemotherapy involving a combination of drugs may be prescribed. Commonly used drugs are adriamycin, vincristine, cytoxan and cisplatin. Cisplatin and adriamycin are the most effective. A recent study has shown increased survival time using surgery and chemotherapy as opposed to surgery alone.
Length of survival is improved with the aforementioned combination of surgery and chemotherapy. If metastasis to the lungs has occurred, prognosis will be poorer.
MODE OF INHERITANCE:
No clear evidence for genetic transmission of osteosarcoma exists. Some breed lines of larger breed dogs are apparently more susceptible than others to osteosarcoma.
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