Osteosarcoma (OSA) is the most common bone tumor in dogs accounting for nearly 85% of all bone tumors. There are two forms of OSA: 1) the appendicular form which occurs in the limbs and 2) the axial form which occurs within the skull, ribs, and pelvis. The incidence of this cancer in dogs is about 1:10,000. OSA most commonly occurs in large, middle-aged dogs weighing more than 40 pounds. The most common breeds include Saint Bernard, Great Dane, Golden Retriever, Labrador retriever, and Rottweiler. The appendicular form of OSA typically occurs near the wrist, shoulder, or knee. The cause of this cancer is unknown but based upon common breeds, there appears to be a genetic component. Other proposed causes include microscopic injury to bones in young growing dogs, metallic implants, and trauma.
The most common history associated with this cancer includes lameness that is nonresponsive to anti-inflammatory medications. On physical exam, swelling is often noted along the affected limb. Radiographs of the lesion aid in determining whether a bone tumor exists and destruction of the bone is usually noted. However, this does not diagnose a tumor. Diagnosis can only be accomplished via a bone biopsy or aspirate. As part of staging for this cancer, radiographs of the chest are taken. In 85% of dogs, the chest radiographs are normal indicating no obvious signs of spread (metastasis). In over 90% of dogs with appendicular OSA, metastasis is likely present but is too small to be visualized with radiographs. Computed tomography (CT), commonly used to scan the lungs in people, has been used in veterinary medicine and appears more sensitive in detecting metastasis in dogs than radiographs. The chance for metastasis for tumors in the axial location is <50%, therefore radiographs at generally sufficient for staging without the need for a CT scan of the lungs. Routine blood work (CBC and serum chemistry) is also performed in order to ensure favorable overall health of the patient prior to surgery as well as evaluate the enzyme alkaline phosphatase (ALP) which has been shown to be prognostic for this cancer.
Prognostic factors for this tumor include:
Dogs with large tumors (poor)
Dogs with visible cancer spread (poor)
Dogs with high ALP (poor)
Dogs receiving chemotherapy (good)
Older or very young dogs (poor)
Tumor location/metatastasis (tumors on the limbs have a high rate of metastasis)
Anatomic tumor location (tumors arising from the ribs have a more aggressive behavior)
Tumor removal (non removal or incomplete removal is poor)
As far as treatment for this cancer, surgery is the mainstay of therapy for OSA and provides immediate relief of pain associated with this cancer. Prior to surgery, an orthopedic exam is necessary to ensure the patient has no concurrent orthopedic disease that would prevent an amputation if the tumor is appendicular. However, due to the aggressive nature of this cancer, surgery alone provides little long-term control. With surgery as a sole therapy for appendicular OSA, studies have shown that dogs develop cancer spread within 3-4 months and <10% of dogs will live 1 year. This is due to the fact that most dogs have micrometastatic disease at the time of presentation that proliferates once the primary tumor has been removed. Because of this, chemotherapy is recommended post-op. If the tumor is axial, the need to consider chemotherapy is contingent on the tumor grade and completeness of surgical removal. Chemotherapy is designed to kill cells that are rapidly growing such as the metastatic disease and residual disease at the tumor site. Radiation therapy is also used to treat incompletely excised axial tumors and is generally more effective at effectively treating the residual cancer cells at the surgery site.
Several chemotherapy agents have been used, including Adriamycin, Cisplatin and Carboplatin, as all appear to have similar efficacy against this tumor. Combination protocols utilizing these agents are designed to limit drug resistance by virtue of alternating between more than one drugs. However, studies have shown mixed results and it appears that the best combination protocol is not known; therefore, the current “gold-standard” is to administer 6 doses of Carboplatin single-agent therapy. With the addition of chemotherapy, studies show >50% of dogs will live one year or longer when treating the appendicular form of OSA. For rib OSA, 50% of dogs survive 8 months with surgery and chemo. The important point is that dogs maintain an excellent quality of life during therapy. For axial (skull and backbone) OSA the average survival time is 4-5 months, although in cases where the tumors affect the upper jaw, prognosis may be far better (1+ years with aggressive therapy). For OSA occurring in the soft tissues outside of the bone, the average survival time is 1-3 months, but this typically doubles when chemotherapy is used in conjunction with surgery.
In patients where surgery is not possible and pain relief is needed, radiation therapy may offer palliation, which is relief of bone pain without curing, in 80% of patients. This involves 4 treatments (1 x/week) of radiation therapy with minimal side effects. However, activity must be moderately restricted because the decrease of bone pain from the radiation therapy could cause most dogs to attempt to increase their activity, which could potentially lead to breaking the bone where the tumor is present. When palliative radiation therapy is given, chemotherapy may be given concurrently and has been shown to yield survival times around 6-8 months for 50% of the dogs; without chemo, survival time is generally around 4-5 months.
There are additional treatments that may be used in conjunction with surgery, radiation therapy, and chemotherapy. Non-steroidal anti-inflammatory drugs (NSAIDs), such as deramaxx, rimadyl, or feldene, have been shown to have anticancer effects against some types of carcinomas; however, limited data exists on the use of these drugs in treating thyroid tumors. Also, starting dogs on low-dose thyroid supplementation may be beneficial. A newer form of therapy that combines an NSAID with low-dose chemo is known as metronomic therapy. This form of treatment is designed to be anti-angiogenic, or against new blood vessel formation. Cancer cells must acquire new blood vessels for oxygenation and nourishment in order to grow into a sizable mass. The goal of starting metronomic chemo is to prevent these blood vessels from forming to feed any microscopic cancer cells. This may thereby delay disease recurrence and improve the long-term prognosis.