OSTEOSARCOMA

About the Diagnosis

Cause: Osteosarcoma is a painful, life-threatening cancer of bone cells. This type of cancer commonly develops in limbs (appendicular skeleton, meaning the front or back legs) of large to giant breed dogs and much less commonly in small dogs. Most dogs that have osteosarcoma are 7 years of age or older. Neutered dogs are affected more often than unneutered dogs for a reason that is not known. Although the exact cause is unknown, osteosarcoma may be related to previous fractures and other bone diseases in some cases.

Osteosarcoma is the most common primary cancer of bone in dogs. This means that the cancer arises in the bone cells. It is a very aggressive type of cancer and has a tendency to travel from the bones in the legs to distant parts of the body (metastasize). The cancer cells metastasize most commonly to the lungs and bones not in the legs (axial skeleton, such as pelvis or vertebral column) through the bloodstream. They can also metastasize to nearby lymph nodes. If metastasis has occurred, the outlook (prognosis) is worse.

Note: Although primary bone tumors are rare in cats, osteosarcoma is the most common type. In cats, this cancer does not usually metastasize, and removal of the tumor through surgery (often requiring limb amputation) can be permanently curative.

Symptoms: Because the primary disease affects bones of the limbs, the most common symptoms include lameness (also called limping or favoring a leg) and a hard swollen lump on a front or rear leg. There is usually a general weakness that may be accompanied by a decreased appetite and weight loss. The dog may no longer be interested in common daily activities because this can be very painful. Later in the disease process, if metastasis to the lungs occurs, breathing difficulty may develop. It is important to note that there are many other, less serious causes of these symptoms, and it should not be assumed that osteosarcoma is present simply because some or even all of these symptoms are present. The only way to be sure about a diagnosis of osteosarcoma is through testing, such as with a biopsy.

Diagnosis: The diagnosis is based on symptoms, the history, and results of several tests. Your veterinarian will take a complete medical history, meaning asking you about the duration and extent of symptoms, any medications and their effects, and so on, and also perform thorough physical and orthopedic exams to assess the possibility of other disorders masquerading as osteosarcoma. Because other diseases can mimic osteosarcoma, several tests may be needed to reach a diagnosis. X-rays of the affected area are taken to help differentiate a primary bone tumor like osteosarcoma from other diseases of the bone. X-rays of the lungs, palpation (feeling with the fingertips) of lymph nodes, and even sophisticated tests such as nuclear scintigraphy (bone scan) can help to determine if the tumor has spread or metastasized. Nuclear scintigraphy is a special procedure that is used to detect very early changes in bone diseases. This technique is performed by a veterinary specialist. Routine blood tests such as a complete blood cell count (CBC) and biochemistry profile and a urinalysis are performed to assess the overall health of your pet and to help determine the severity of osteosarcoma when it is present. A biopsy of the affected bone, which is taken while the patient is under general anesthesia, is the definitive test for osteosarcoma.

Living with the Diagnosis

Osteosarcoma is a malignant, incurable cancer. Therefore, when osteosarcoma is confirmed, many difficult questions are raised regarding quality of life and life expectancy. Once an animal has osteosarcoma, he/she is more likely to die of osteosarcoma than anything else. Survival times are usually on the scale of months, with greater survival times and quality of life occurring in patients who undergo treatment (see below). Occasional dogs (<1%) have had their cancer shrink and disappear spontaneously and have survived their normal life span despite having had a proven osteosarcoma.

Although it may be difficult, it is important to prevent your dog from participating in activities that may increase pain and discomfort, such as playing and walks, when osteosarcoma is present but has not been removed (no surgery).

The bone of an osteosarcoma tumor is very brittle, and exercise in a patient with osteosarcoma increases the possibility of a bone fracture at the tumor site, also known as a pathologic fracture.

Treatment

The main concern with osteosarcoma is pain at the site of the tumor. This is why most owners of patients with osteosarcoma are faced with a decision involving one of three choices: performing surgery to remove the tumor (most commonly, amputation of the affected leg), leaving things as they are and giving medications to alleviate pain, or humane euthanasia (putting the animal to sleep with a lethal injection). None of these is an easy or even acceptable option. For example, even with medication to help control pain, an animal with osteosarcoma may not be completely pain free if the tumor is not removed. However, the choice of doing nothing and letting the disease take its natural course is likely to be a painful and slowly life-threatening one. It is useful to remember that surgery such as amputation, as difficult as it may sound, is generally an enormous relief to patients with osteosarcoma. The pain of this tumor often has made them unable to use the affected leg properly anyway, such that they have already been walking on only three legs and surgically removing the leg (amputation) simply removes the source of pain. Dogs and cats manage extremely well on three legs; the only common exception is if they have very severe arthritis or other reasons for pain or weakness on the three legs.

Several types of treatment are available. Your veterinarian will discuss the most appropriate options for your dog and together a decision can be reached. Not all treatment options are recommended for all dogs with osteosarcoma. Pain medication (e.g., nonsteroidal anti-inflammatory drugs and opioids) can be given as a form of palliative pain control. Radiation therapy may improve the quality of life and slow down the progression of the tumor but will not reverse it. Surgery is the most widely used and recommended option. Removal of the tumor through amputation of the affected limb is most commonly performed. Alternatively, a limb-sparing procedure may be performed in which only the affected part of the bone is removed and replaced with transplanted bone from a bone bank. If your dog is a candidate for this limb-sparing procedure, your veterinarian will recommend a veterinary surgical specialist because only a few veterinary centers around the world perform this type of procedure. Surgery is most effective when combined with chemotherapy to control microscopic spread to other organs (metastatic disease). Chemotherapy does not offer much benefit unless surgery or radiation therapy is also performed. Chemotherapy is given as periodic (weekly or every 3 weeks) injections. It does not cause a pet's hair to fall out, and although side effects are possible with any drug including chemotherapy drugs, these occur only in a minority of cases. The goal of chemotherapy is to slow the progression of the tumor and its spread; only very rarely does chemotherapy (even if combined with radiation therapy and/or surgery) eliminate osteosarcoma altogether.

It is very difficult to predict the exact survival time of a dog with osteosarcoma because there is wide variation. Many factors must be considered including the general health of the dog before the diagnosis, metastasis of the disease to other parts of the body, and the response to treatment.

DOs

Before the Diagnosis:

• Do seek a second opinion if you are not confident of the diagnosis you were given. Osteosarcoma cannot be diagnosed without a biopsy.

After the Diagnosis:

• Try to keep your pet as comfortable as possible; osteosarcoma can be a very painful condition.

• Realize that the result of amputation is the elimination of chronic, ongoing pain and that the use of a leg with osteosarcoma is already compromised or lost even before amputation.

• Accept that over the long term amputation often seems more disturbing to us than to the animals themselves and that virtually all dogs and cats circulate perfectly well on three legs.

• Inform your veterinarian if your pet has ever been diagnosed with a medical condition and is taking medication.

• Give medication exactly as directed by your veterinarian, and if you are concerned about possible negative effects, discuss them with your veterinarian immediately rather than simply discontinuing the treatment.

DON'Ts

Before the Diagnosis:

• Do not delay seeking veterinary attention if your dog is limping/favoring a leg and has localized swelling in a limb, even if pain is not apparent. Osteosarcoma is one possible cause, and it should be investigated.

• Do not force your dog to exercise if he or she appears painful and is reluctant to do so.

After the Diagnosis:

• Do not give medications intended for humans because some of these drugs may not be appropriate and may even interfere with some treatments.

When to Call Your Veterinarian

• If you cannot keep a scheduled appointment or if you are unable to give medication as directed.

Signs to Watch For

• General signs of illness: lethargy, weakness, decreased appetite, aggressiveness, and other behavior changes.
• Signs of osteosarcoma: lameness, swelling and pain in a limb, and respiratory difficulty.

Routine Follow-Up

• Follow-up visits will be scheduled to assess your dog's response to treatment and to closely monitor for potential complications.