Dog Bone Cancer (Osteosarcoma)

Introduction

Osteosarcoma is a common type of bone cancer that develops from cells of the bones in domestic dogs and cats. It is fairly rare in other species. There are other types of cancer that can form from bone, including chondrosarcoma, fibrosarcoma and hemangiosarcoma. However, osteosarcoma is by far the most common primary bone cancer in dogs. Primary bone cancer/tumors are those that are caused specifically by overgrowth of bone cells. They are different from secondary bone tumors, which form from cells of other organs or tissues and then metastasize – or spread – to bone.

Osteosarcoma initially involves the uncontrolled or out-of-control growth of osteoblasts - immature cells that are the building blocks for mature bone. When a dog has osteosarcoma, osteoblasts multiply without any of the constraints that normally control bone growth. As a result, abnormal bone forms in the affected areas, causing hard, swollen, painful masses. In dogs, osteosarcoma often develops near the joints of the legs. These are the elbow and wrist (carpus) in the front legs, and the knee (hock) and ankle (tarsus) in the hind legs. Osteosarcoma is malignant, which means that it progressively worsens over time and ultimately will be fatal. This type of cancer usually spreads very quickly in dogs, especially from bone to the lungs. Even when cancerous cells migrate from the bone and multiply in other areas of a dog's body, the condition is still considered to be osteosarcoma, or primary cancer of the bone. Large and giant breed dogs develop osteosarcoma much more frequently than do smaller breeds.

Causes & Prevention

Causes of Osteosarcoma
The most common sites of osteosarcoma in dogs are the long bones of the legs. This particular form of cancer frequently occurs in the upper long bone of the front leg that connects the shoulder to the elbow, or in one of the two lower long bones of the front leg that connect the elbow to the wrist. These bones are called the humerus, and the radius and ulna, respectively. Other common sites of osteosarcoma are the femur (the upper long bone of the hind leg that connects the hip to the hock), and the tibia (one of the two lower long bones of the hind leg that connect the hock to the ankle). When a dog's legs are affected by osteosarcoma, the condition is called "appendicular" osteosarcoma. Osteosarcoma can also affect other parts of a dog's skeleton, including the vertebrae, the upper jaw (maxilla), the lower jaw (mandible), other facial bones and the flat bones of the ribs. This is referred to as "axial" osteosarcoma.

The causes of osteosarcoma in dogs are not well-understood. Some authorities hypothesize that minor-to-moderate repetitive trauma to the weight-bearing bones of the legs of rapidly-growing large and giant breed puppies might predispose them to developing osteosarcoma at the sites of those injuries. However, this theory has not been proven. Dogs that have had broken bones (fractures), surgical orthopedic implants or other bone disorders seem to develop bone cancer more frequently than do other dogs.

Prevention of Osteosarcoma
There is no realistic way to prevent osteosarcoma from developing in companion dogs. Certainly, trauma to skeletal bones – especially the long bones of a dog's legs - should be avoided. Other ways to promote a long, healthy, disease-free life are to always feed a high-quality diet, have fresh water freely available and take the dog to a veterinarian regularly for check-ups and vaccination boosters. Routine blood work (a complete blood count and serum biochemistry profile), a urinalysis and a thorough physical examination, done annually, can really help to identify medical diseases or disorders, including those of the kidneys, liver, adrenal glands, heart, lungs, gastrointestinal tract, thyroid gland or other vital organs, even before the owner notices any symptoms of illness or injury. Early identification of medical disorders is one of the best ways to improve dogs' chances of recovery or cure. However, until the actual causes of osteosarcoma are discovered, this disease will to continue to be a thorn in the side of pets and their people.

Special Notes
Osteosarcoma accounts for about 85% of all primary bone tumors in dogs (tumors that originate from bone cells). Osteosarcoma is much less common in companion cats, although it does occasionally occur in our feline friends.

Symptoms & Signs

How Osteosarcoma Affects Dogs
Osteosarcoma frequently affects the upper or lower long bones of a dog's front legs. These are called the humerus, and the radius and ulna, respectively. The long bones of the rear legs can be affected, as well; these are the femur, tibia and fibula. The jaw and other facial bones, as well as the ribs and vertebrae, can also be subject to osteosarcoma. The first sign of osteosarcoma usually is a slight but progressive limp in an adult dog that has no prior history of trauma or injury. Over time, the areas where the bone tumors are located will become swollen, hard and extremely painful.

Symptoms of Osteosarcoma
Dogs suffering from osteosarcoma may develop one or more of the following symptoms, depending upon the location and maturity of their bone tumors:

Lameness; progressive and of varying severity; the dog may be non-weight-bearing on the affected leg; limping is typically noticed by the owner of a mature dog with no history of leg trauma
Pain (often severe)
Lethargy
Weakness
Swelling around the affected area of the leg; most often evident in one leg rather than several; usually painful
Broken bones at the tumor site (called "pathologic fractures"); caused by bone weakness from the cancer; usually no history of physical injury to the area
Swollen upper jaw (maxilla); usually painful
Swollen lower jaw (mandible); usually painful
Pain when opening the mouth
Nasal discharge
Swelling and pain along the spine
Swelling and pain around the ribs
Difficulty eating or chewing (dysphagia)
Lack of appetite (inappetence; anorexia)
Respiratory distress (difficulty breathing; dyspnea); typically caused by rapid spread [metastasis] of the cancer from bone to lung tissue; can also be caused by osteosarcoma of the ribs)
Dogs at Increased Risk
Osteosarcoma is basically a disease of middle-aged and older dogs, with the average age of onset being about 8 years. Males and females are equally at risk. Large and giant breed dogs are predisposed to developing this type of cancer, although the reasons for this association are not well-understood. The Saint Bernard, Newfoundland, Great Dane and Great Pyrenees are reported to be 60 times more likely to develop osteosarcoma than are dogs that weigh less than 65 pounds. Large breed dogs, such as the Irish Setter, Golden Retriever, Labrador Retriever and Boxer, reportedly are about 8 times more likely to be affected by osteosarcoma than are smaller dogs. Toy breeds rarely develop this type of bone cancer.

A dog's height and weight are more closely associated with its chances of developing osteosarcoma than is its breed. Larger animals usually develop osteosarcoma in only one of their legs. Smaller dogs can develop osteosarcoma in their legs, but they are equally likely to have this form of cancer show up in other parts of their skeleton.

Diagnosis & Tests

How Osteosarcoma is Diagnosed
Osteosarcoma is preliminarily diagnosed on the basis of a dog's history, a thorough physical examination and X-rays (radiographs) of the suspected affected areas of bone. If osteosarcoma is present, the X-ray films will show aggressive bony lesions characterized by a moth-eaten, ragged appearance. Osteosarcoma is a very aggressive form of cancer. It almost always spreads quickly through the blood stream and lodges in remote places in the dog's body – especially in the lungs. This is referred to medically as "metastasis." Once bone cancer is identified, the veterinarian will probably recommend taking X-rays of the dog's chest, to assess whether the cancer has spread from bone to the lungs. When analyzing these films, the veterinarian will be looking for one or more hazy but fairly distinct whitish patches throughout the tissues of one or both lungs. Actually, the doctor will hope not to find these shadowy patches on the X-ray films. Unfortunately, roughly 95% of dogs with osteosarcoma will ultimately have their cancer spread to their lungs or to other areas of their bodies, even if their primary bone tumors are surgically removed.

The initial medical database probably will also include a urinalysis and routine blood work (a complete blood count and a serum biochemistry profile). The results of these tests will help the veterinarian rule out a number of other possible causes of the dog's symptoms. However, a definitive diagnosis of osteosarcoma requires microscopic evaluation of tissue samples taken by surgical biopsies of affected bony areas. Depending upon the location of the tumors and the owner's willingness to treat the condition aggressively, the attending veterinarian may suggest surgically removing the entire mass, along with quite a bit of the surrounding tissue, to get diagnostic samples, and hopefully at the same time to possibly remove most or all of the cancerous bone cells. Because most cases of osteosarcoma start in one of a dog's legs, removing the cancer typically is attempted by partial or total amputation of the affected limb. The biopsy sample, or the part of the leg that was surgically removed, will be sent to a medical laboratory for evaluation by a veterinary pathologist who will be able to identify the exact kind of cancer that is present.

If the attending veterinarian suspects that her patient has osteosarcoma in places other than its legs, a computed tomography scan (CT scan) will probably be advised. The detailed images produced by a CT scan can help the doctor assess the severity and spread of the cancer and plan surgery, radiation, chemotherapy or other forms of therapy. CT scans are highly specialized diagnostic procedures that require expensive medical instruments and well-trained personnel. They usually are only available at veterinary teaching hospitals and some referral centers.

Special Notes
Chondrosarcoma, which is the second-most common form of primary bone cancer in dogs after osteosarcoma, usually affects the ribs, pelvic bones or muzzle rather than the long bones of the legs. Chondrosarcoma typically shows up in middle-aged dogs as a large, hard, painless mass in an area where cartilage is present. Cartilage is a gristly type of connective tissue that provides the matrix or framework upon which bone is formed. In dogs, chondrosarcoma is much less aggressive than osteosarcoma.

Treatment Options

Treatment Options
The goals of treating a dog that has osteosarcoma are to relieve pain and prolong the pet's disease-free quality of life for as long as realistically possible. Because osteosarcoma tends to spread rapidly, it should be taken very seriously and treated aggressively. The lungs are often one of the first places that osteosarcoma migrates to. X-rays (radiographs) of a dog's chest may be recommended before it is placed under anesthesia to undergo surgery, in order to determine whether the cancer has spread to its lungs and, if it has, how advanced the dog's disease has become. If any of the lymph nodes are swollen, the veterinarian may take samples using a fine needle aspirate procedure. This technique involves inserting a sterile needle into the suspicious lymph nodes and withdrawing a tiny amount of tissue, fluid and cells from those nodes by pulling back on the attached syringe. The aspirated samples will be expressed from the syringe onto glass slides and examined under a microscope.

When osteosarcoma affects a dog's leg, the standard course of treatment is surgical amputation. Most dogs handle this fairly well and seem relatively unaffected by having only 3 usable legs. This is especially true if all or part of one of the hind legs is removed, because dogs do not carry as much of their weight on the back legs as they do on the front. New surgical techniques are being developed that may enable veterinarians to save a dog's limb by removing the cancerous portion and sparing the remainder of the leg. These procedures are only available at specialized veterinary teaching hospitals and surgical referral centers. At this time, salvage operations have more complications, such as infections and tumor recurrence, and the same average survival times, as does amputation. Most veterinarians suggest amputating the leg from at least one joint above the affected area. Hopefully, this will relieve the pain associated with osteosarcoma and improve the dog's overall happiness, contentment and enjoyment of life.

Surgery is also the preferred treatment for a dog that has osteosarcoma in its ribs, spine, jaw or other facial bones. All of the affected bones should be removed. Unfortunately, in many cases, complete excision is not possible because of the location of the tumors near vital blood vessels, nerves and other key structures.

Radiation and chemotherapy can be used as adjuncts to surgery in dogs with osteosarcoma. The protocol for chemotherapy usually involves giving intravenous drugs every few weeks for a set period of time. This must be done in a hospital, usually at a referral center under the supervision of veterinary cancer specialists called oncologists. Chemotherapeutic agents are very potent. Using them can be unpleasant and even dangerous for the dog. Basically, these drugs enter the dog's bloodstream and circulate throughout its body, targeting and killing rapidly-dividing cells, including cancer cells. However, chemotherapy drugs also target a number of normal rapidly-dividing cells, like those lining the stomach, intestines and hair follicles. Nausea is a common consequence of chemotherapy, as is hair loss. Chemotherapy may increase the survival time of dogs with bone cancer, but it rarely accomplishes a complete cure. Because osteosarcoma is so aggressive, it almost always will have spread by the time it is diagnosed. Owners of dogs with osteosarcoma will want to consult with a veterinary oncologist before embarking upon a course of surgery, radiation or chemotherapy.

Some owners decide not to pursue any of these treatments, for a variety of different personal reasons. Their dogs can still benefit from good supportive care for the rest of their lives. Oral pain medications (analgesics), such as non-steroidal anti-inflammatory drugs (NSAIDs) and opioids, can be helpful. Prescription pain-relieving patches, which slowly release small amounts of potent anti-pain drugs directly through the dog's skin, are also available. Sadly, many dogs with osteosarcoma are euthenized shortly after their diagnosis is confirmed. This is a realistic option for some owners. Osteosarcoma is a painful and potentially fatal disease. Chances are good that it has already spread from bone to other tissues by the time it is discovered in our companion dogs.

Prognosis
Most dogs with osteosarcoma will eventually succumb to the effects of their disease, with or without treatment. Dogs with bone cancer in one of their legs typically live only 4 or 5 months after that limb is amputated; roughly 10% will survive up to a year after surgery. When amputation is combined with chemotherapy, the average survival rate increases to about 6 to 12 months, at the most. If no treatment is done, dogs with osteosarcoma usually only live for a month or two after their disease is diagnosed.