Canine 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 pain-killers 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.

If your vet suspects that your pet has a bone cancer they will first want to take X-rays of the affected site. Whilst osteosarcoma cannot be definitively diagnosed on an X-ray alone, a presumptive diagnosis can be made and in many cases biopsy is not necessary. In addition X-rays of the lungs are obtained to see if the cancer has spread. If there are any other painful bone lesions these should be investigated. If your vet 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 clearly 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-conserving 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 in the same bone 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-conserving surgery are not appropriate, palliative therapy can be administered in the form of radiotherapy (weekly on 3 out of 4 weeks) and chemotherapy in conjunction with the first and final doses of radiation. Other radiotherapy treatment protocols are described and this is an active area of research so it would pay to consult an oncologist if further detail is required.

A further novel therapeutic approach is the use of a drug that is very highly concentrated in active bone tissue, as one finds in osteosarcoma. At the high drug concentrations achieved in a bone cancer the drug is supposed to become toxic, specifically targeting the nearby cells which are of course cancerous ones. In practice, this approach has failed to achieve the results that were initially promised. There remain isolated cases that exhibit tremendous responses so this treatment option should not be rejected completely. It may be that further refinements to the treatment plan in the future result in improved cancer control.

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 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.

Following diagnosis, life expectancy 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 6½ months with a few patients alive after 2 years.
  • Patients receiving palliative radiation and chemotherapy have an average life expectancy of six 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.

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 the veterinary oncologist.