Osteosarcoma

An osteosarcoma is cancer of the bone. It usually arises in the bones of the limbs but can develop in the bones of the skull, spine or ribcage and there are rare cases of this cancer arising in non-bony tissues like mammary glands and muscle.

Osteosarcoma is most commonly found in large or giant breeds of dog, and it is well known that certain dog breeds develop this cancer more often than others. It is frequently seen in Rottweilers and Irish wolfhounds in particular.

Osteosarcoma of the limb bones can be extremely painful and the typical presenting complaint for affected dogs is that of an intermittent lameness. The lameness may respond to standard doses of painkillers initially but rarely for more than a week or so. Sometimes a swelling in the bone at the site of the cancer may be noted and this is often painful, red and hot to the touch. Rarely, a pathologic fracture (i.e. where the bone breaks because it is too weak due to cancer) may occur and non-weight baring lameness with severe acute pain would develop.

If your veterinarian suspects that your pet has a bone cancer they will first want to take X-rays or a CT scan of the affected site. Whilst osteosarcoma cannot be definitively diagnosed on an X-ray alone, a presumptive diagnosis can be made and in some cases biopsy is not necessary. However, when the bone is severely destroyed, a needle can be inserted in the area that is suspicious of cancer to obtain a cell sample. In addition, X-rays or CT scan of the lungs are obtained to see if the cancer has spread. If there are any other painful bone lesions these should be investigated as osteosarcoma can spread to other bones. If your veterinarian detects any enlarged lymph nodes in the region of the bone lesion samples can be taken using a needle and syringe to collect a few cells for examination.

While the most pressing concern in patients with osteosarcoma is undoubtedly the pain associated with the bone cancer, it is also important to be acutely aware of the fact that osteosarcoma in dogs spreads rapidly via the blood stream. Therefore any treatment plan needs to address the secondary spread of the cancer as well as the bone cancer itself. Therefore, management of osteosarcoma in dogs focuses on both the primary and the secondary tumours.

Treatment: Primary Tumour
The ideal therapy involves complete resection of the bone cancer and this most often requires limb amputation. Many dogs cope extraordinarily well following amputation but there are some patients for whom this would be inappropriate. Concurrent neurological problems or severe arthritis are reasons for not considering amputation. Moderate arthritis can be managed extremely well with suitable medication and need not be a reason for rejecting amputation; obviously this should be evaluated on a case by case basis. Amputation invariably has a tremendous impact on the state of mind of the patient. While it is clearly a major operation, these patients seem to be so relieved to be free from the pain that they recover extremely quickly.

In the event that amputation is considered inappropriate there are other options. For osteosarcomas of the distal radius (this is the lower front limb, just above the wrist joint equivalent in dogs) an operation can be performed in which the affected piece of bone is removed and replaced by a special implant. This procedure, called limb-sparing surgery, can restore the dog to normal mobility in a very short time. While this approach carries a definite appeal, it is critical to emphasise that the degree of cancer control afforded by this approach can at best be equivalent to amputation. Following this treatment there is also a risk of the cancer returning at some point. More importantly, this procedure is associated with a high risk of complications, worst of which is infection associated with the metal implants.

For the patients in which amputation and the so-called limb-sparing surgery are not appropriate, palliative therapy can be administered in the form of radiotherapy (weekly for 4 weeks or as a quad shot over 2 consecutive days) and possibly chemotherapy. Other radiotherapy treatment protocols are described and this is an active area of research so it is recommended to consult a radiation oncologist if further detail is required. A drug called bisphosphonate, that helps to slow down bone destruction, and strong analgesia (pain relief) are recommended alongside radiotherapy in the palliative setting.

Immunotherapy is being developed against osteosarcoma and there is hope it will be part of a multimodality treatment in the future.

Treatment: Secondary Spread
Sadly, despite control of the original cancer, it is the development of metastatic cancer (that has spread elsewhere in the body) that leads to euthanasia of many patients with osteosarcoma. Optimal outcomes are achieved by treating both the primary and the secondary cancers. In order to treat all the sites to which the cancer may spread it is necessary to give chemotherapy. As a rule this treatment is tolerated extremely well. The most widely used chemotherapy protocol involves treatment once every three weeks for a total of four to six doses. Over the last few years veterinary oncologists have tested novel combinations of chemotherapy agents for the management of secondary cancer in osteosarcoma. Despite increasing levels of side effects, there remains no good evidence of an improvement in outcome associated with these treatment protocols. With the constant emphasis on quality of life, most oncologists choose to use chemotherapy aiming for reduced side effects whilst maintaining the same beneficial results in terms of cancer control and overall survival. In most cases, chemotherapy treatment passes uneventfully. All owners must be fully apprised of the risks of chemotherapy administration prior to embarking on a course of treatment.

For dogs fulfilling specific criteria (primary cancer under control for longer than 300 days, only 1 to 2 slow growing nodules in the lungs and no other site of spread), surgical removal of lung spread may be considered as it has been shown to improve survival.

Following diagnosis, life expectancy for osteosarcoma of the limbs can be summarised as follows:

• Without therapy the average survival time is approximately 2 months. This is primarily determined by the discomfort associated with the primary cancer.
• If amputation is performed the average survival time is increased to 3 to 6 months.
• Patients receiving palliative radiation and chemotherapy have an average life expectancy of 4 to 6 months.
• Amputation (or limb-conserving surgery) with chemotherapy makes the average survival time just a little less than one year with 2 in 10 dogs still enjoying a good quality of life 2 years after surgery.

The prognosis for osteosarcoma of the skull is 8 months on average, but more specific locations such as the mandible have a longer outcome.

The average prognosis for osteosarcoma of the rib is 3 months after surgery and 8 months when surgery is combined with chemotherapy.

The prognosis for osteosarcoma arising outside the bone is guarded with an average survival of 1 months after surgery only and 5 months when surgery is combined with chemotherapy.

Not all of the treatment options outlined are suitable for all patients. While it is important for owners to be aware of the options and the reasons for choosing one treatment over another, the ultimate decision about which treatment plan is most appropriate is best made in conjunction with a veterinary oncologist.