Bone Tumors
General Information
Bone tumors may be non-cancerous (benign) or cancerous (malignant). Benign (non-cancerous) tumors are those that do not spread to other parts of the body; these tumors are the less dangerous type. Malignant (cancerous) tumors can spread and are much more dangerous. The quickest, most accurate and least painful method of definative diagnosis of the tumor type is to biopsy the tumor.
Bone Cancer: Osteosarcoma
What is osteosarcoma?
Osteosarcoma is a cancer of the bone. Osteosarcoma may develop in dogs or cats. It is the most common primary bone tumor seen in dogs. A primary tumor is one that develops first in a particular location and not as a result of spread (metastasis) from cancer in other body tissues. The most common sites for osteosarcoma are the skull and limbs. The backbones (vertebrae) and ribs also may be affected. Osteosarcoma often spreads (metastasizes) to the lungs and other bones. Osteosarcoma in dogs has a poor prognosis because of the high rate of spreading (metastases) to other areas of the body. Osteosarcoma is less aggressive in cats.
What causes osteosarcoma?
The cause of osteosarcoma is unknown; however, repeated minor trauma to the limbs may be involved. Large-breed and giant-breed dogs and domestic shorthair cats are more likely to develop osteosarcoma.
What are the signs of osteosarcoma?
The signs of osteosarcoma will vary, depending on the location of the primary tumor, the size of the tumor, and the presence of metastatic tumors. Swelling and pain often are seen in the affected limb with resulting lameness. The lameness may be mild or the animal may be unable to stand on the affected leg. The tumor may be large enough to be seen or felt (palpated). Signs may be associated with the involved area. As examples, breathing difficulties (respiratory distress) may be associated with a large osteosarcoma of the ribs. If the tumor is in the vertebrae, nervous system signs, such as paralysis, may be present. The affected bone may break (fracture) easily.
How is osteosarcoma diagnosed?
Osteosarcoma is diagnosed by a good history, a thorough physical examination, radiographs (X-rays), and biopsy (removal and examination of tumor tissue). Other diseases, such as bone infections (infectious osteomyelitis), have similar signs and must be ruled out as the cause of the bone lesions. Radiographs of the bone tumor and of the chest are taken to assess the tumor and any spread to the lungs (metastases). A nuclear bone scan can detect bone and lung metastasis at earlier stages than plain radiography. The results of a nuclear bone scan must be interpreted with caution because sites of previous trauma or inflammation can be indistinguishable from cancer. Bone biopsy can be performed under local or general anesthesia, depending on the degree of pain and the animal's behavior.
How is osteosarcoma treated?
Treatment for osteosarcoma is aggressive. The location of the tumor will affect treatment decisions. Amputation of the affected leg followed by chemotherapy is considered the treatment of choice for dogs. Sometimes the tumor can be removed without amputation of a limb (limb-sparing surgery). If the tumor is in the skull, the affected area (for example, the jaw) may be removed with the tumor. If the tumor is in the ribs, the affected part of the chest wall is removed. Chemotherapy is medication that is toxic to cells and should be monitored carefully. Some animals may not be able to tolerate chemotherapy. Side effects, such as nausea and vomiting, often occur but usually can be controlled.
In cats, the same surgery is done but chemotherapy is not necessary due to a low rate of spread (metastatic rate). Sometimes, radiation therapy is done to shrink the tumor and decrease pain. In this case, the radiation therapy is not aimed at a cure, but rather improving the dog's or cat's quality of life.
What is the prognosis for animals with osteosarcoma?
Generally the prognosis (outcome) for animals with osteosarcoma
is guarded. In dogs without treatment, spread (metastasis) to
the lungs or other bones, fractures, and diminished quality of
life develop within 4 months of diagnosis. With amputation alone,
the average survival time is 4 months. With amputation or surgical
removal of the tumor plus chemotherapy, the average survival time
is 1 year. A few dogs may live for 2 years with surgery and chemotherapy.
In cats, treatment with amputation alone can lead to average survival
times of up to 4 years
Most bone tumors are cancerous and are usually painful. Therefore, a prompt decision for surgery is required to relieve discomfort and to increase the chances for a successful outcome.
Important Points in Treatment
1. Some types of bone tumors require considerable personal and economic commitment by you, the owner. The doctor will assist you in any way possible to arrive at an acceptable decision on how far to pursue treatment.
2. A complete physical examination, usually including laboratory tests, radiographs (x-rays) and biopsies (the microscopic examination of tissue samples), may be required for diagnosing and evaluating your pet's condition.
3. Your pet may be referred to a cancer specialist for further evaluation and/or treatment.
4. Surgery, irradiation, chemotherapy or a combination of these are used to treat bone tumors.
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