Osteosarcoma in Dogs

Overview of Canine Osteosarcoma

Osteosarcoma is a type of cancer that typically arises in the bones of the limbs, or the appendicular skeleton. Less commonly, it may occur in the bones of the spine, pelvis, and skull – the axial skeleton. Osteosarcoma is the most common type of bone cancer, and it is estimated to occur in more than 8,000 dogs in the United States each year. Osteosarcoma occurs very rarely in cats.

The cause of osteosarcoma is largely unknown. It is most common in large-breed dogs (over 50 pounds), so its development in some animals may be related to rapid early growth with resultant increased weight and forces being placed on the bones. It has occurred at fracture sites where metal plates or pins were used to repair the bone, suggesting that chronic irritation may be associated with development of this type of cancer. Rarely, it can occur in areas that have been exposed to radiation therapy.

Male animals are affected at the same rate as female animals. Most dogs are 6 years of age or older when they develop this tumor; however, it does occur in animals as young as two years of age.

This is a highly metastatic, meaning it spreads to other parts of the body, and life-threatening form of cancer and usually causes lameness and general debilitation of your pet during its development and progression. Average life expectancy in dogs that receive treatment in the form of amputation, or surgical removal of the leg, and chemotherapy is 10 to 12 months. Without treatment, life expectancy is usually two to four months. When this cancer affects the axial skeleton, the prognosis is usually worse but is dependent on the site and on the type of surgery and follow-up care. Some animals with tumors of the lower jaw can do well for a year with only surgery.

What to Watch For

Signs of Osteosarcoma in Dogs may include:

• Lameness
• Pain of any of the bones
• Broken bone with minimal trauma
• Swelling of a limb
• Difficulty breathing
• Coughing
• Exercise intolerance or reluctance or inability to exercise normally

Diagnosis of Osteosarcoma in Dogs

Diagnostic tests are needed to recognize osteosarcoma and exclude other diseases that may cause similar symptoms. Tests may include the following:

• Complete medical history and physical examination
• Radiographs (X-rays) of the affected bone
• Radiographs of the chest/lungs to look for metastasis
• Complete blood cell count (CBC)
• Biochemistry profile
• Urinalysis
• Biopsy of the tumor
• Bone scan, which is a specialized type of X-ray that looks for spread of the cancer to other bones

Treatment of Osteosarcoma in Dogs

In addition to administration of pain medications, treatment for osteosarcoma may include the following:

• Surgical removal of the tumor, which usually involves limb amputation
• Chemotherapy, usually done in conjunction with amputation or limb-sparing procedures
• Radiation therapy as an alternate form of palliative treatment for pain relief in very select cases
• Limb-sparing surgery, a type of surgery where the bone containing the tumor is removed and replaced by a donor bone. This procedure is performed only at limited veterinary surgical referral centers.

Home Care and Prevention

Your veterinarian can prescribe pain medications to make your dog more comfortable. You may be asked to administer pain medications to your dog until definitive therapy can be done, but do not give your dog any pills that have not been prescribed or recommended by a veterinarian. At the time of surgery a narcotic pain patch may be placed on the skin. These patches release a constant level of pain medication that is absorbed through the skin.

You should limit your pet’s activity to prevent further pain and to reduce the likelihood of a pathologic fracture, which is an abnormal breaking of the bone due to the cancer weakening it, prior to definitive therapy. Your dog should not run, jump or play during this time and you should watch him carefully or give him assistance when he is climbing stairs or getting in and out of a car.

There is little that you can do to prevent bone cancer from occurring in your pet because the cause is poorly understood. Have your veterinarian evaluate your pet for any lameness that develops. Most forms of lameness are likely to be associated with arthritis or injury to ligaments and tendons. If your dog is not getting better with rest and anti-inflammatory drugs, radiographs of the affected region should be taken to exclude the presence of a bone cancer as a cause of lameness.

In-depth Information on Osteosarcoma in Dogs

Lameness is a general term used to describe pain or discomfort experienced when your dog moves normally or exercises minimally. Although your dog may develop lameness due to arthritis, ligament or tendon tears, or cartilage injury, lameness is also a cardinal sign of osteosarcoma. Therefore, any unexplained or chronic lameness in your pet warrants further investigation.

Some tumors arise from tissues associated with bone and can mimic osteosarcoma. These include:

• Chondrosarcoma (cancer that arises from cartilage)
• Fibrosarcoma (cancer that arises from fibrous connective tissues)
• Synovial cell sarcoma (cancer that arises from the cells that line the joints)
• Hemangiosarcoma (cancer that arises from the blood vessels)

These tumors are far less common than osteosarcoma and tend to affect the axial skeleton (skull, ribs and spine) more often than osteosarcoma.

Other causes of lameness that must be considered and potentially differentiated from bone tumors include:

• Pathologic fractures. If your pet experiences a fracture with minimal trauma, a pathologic fracture should be considered. Although fractures are most often a result of trauma, they can also occur in bones that have been weakened by cancer. Evaluating an X-ray may lead to the suspicion that the bone is abnormal; however, definite diagnosis of a tumor requires a biopsy. Pathologic fractures will not heal if fixed using standard techniques.
• Osteomyelitis (infection in the bone). This is an uncommon condition that occurs as a result of infectious organisms such as bacteria or fungi getting into a bone. These organisms most commonly gain entrance to the bone through an open wound, open fracture or rarely through the blood. The appearance of osteomyelitis may be similar to some bone cancers because it often appears as a proliferative or fuzzy mass-like lesion on an X-ray. Differentiating these types of infection from bone cancer typically requires that a biopsy and a culture be performed.
• Bone infarction. Bone infarction is a very rare condition in which a blood clot blocks the blood supply to a bone, resulting in death of the bone. This appears as a lytic lesion (loss of bone) on an X-ray, which is similar to the appearance that bone cancer can have.
• Metastasis of tumors to the bone. Very rarely a bone cancer can be due to the metastasis (spread) of cancer from a primary cancer elsewhere in your dog. The most common types of cancer that metastasize to bone are mammary gland cancer, multiple myeloma and lymphosarcoma. These types of cancers tend to have a distinctively different appearance on X-rays than osteosarcomas. Although their radiographic appearance may alert your veterinarian to their presence, a biopsy is still required to make a definitive diagnosis. It is important to distinguish tumors that have spread from other tissues to bone from those that arise in bone initially, because the treatment differs. For metastatic tumors, an attempt will be made to find out where the primary (initial site where the tumor arose) cancer is located. If your dog is diagnosed with cancer, your veterinarian may wish to consult with an oncologist (cancer specialist) to fully understand the specific behavior and treatment of the cancer.

Veterinary care should include diagnostic tests and subsequent treatment recommendations.

In-depth Information on Diagnosis

Medical tests are needed to establish the diagnosis of osteosarcoma, exclude other diseases that may cause similar symptoms, and determine the impact of osteosarcoma on your dog. Tests that your veterinarian may wish to perform include:

• Complete physical exam. A thorough exam is necessary to localize the site of the cancer and also to assess your pet’s general health. Your pet’s general condition may drastically influence the treatment that is recommended.
• Radiographs (X-rays) of the affected bone. Although the appearance of the bone on an X-ray may be suggestive of cancer, X-rays are not diagnostic of this condition. There is no risk to your pet in performing an X-ray; however, if your pet is unwilling to lie still long enough to have the radiographs taken, due to pain from the bone cancer or their personality, your veterinarian may recommend that a mild tranquilizer or even anesthesia be administered.
• Radiographs of the chest/lungs. The lungs are the most common location to which bone tumors spread. Therefore, X-rays of the chest should always be taken prior to surgery. Dogs that already have evidence of cancer in the lungs are not good candidates for amputation because their disease is advanced; however, they may be considered for other treatments such as palliative radiation therapy and possibly chemotherapy.
• Complete blood cell count (CBC). This is a test run on blood that counts the number of red blood cells that carry oxygen in the body and white blood cells that fight infection that are circulating in the blood stream. This is a standard test to screen your pet’s general heath and to ensure that it is safe to perform other procedures, such as surgery. However, it does not reveal cancer in the blood.
• Biochemical blood profile. This is a test run on blood to assess the general health of your pet’s organs. It assesses the health of the liver, which filters toxins, creates bile and metabolizes nutrients taken into the body in the food, and function of the kidney, which filters the blood to preserve body water and make urine, and measures levels of electrolytes in the blood. When the measured values differ from values obtained from a pool of healthy animals, or normal values, dysfunction of the organ(s) is suspected. Dysfunction may or may not be associated with the spread of cancer to an organ.
• Urinalysis. These are tests run on a sample of urine. Like the blood tests, it gives your veterinarian information about your pet’s general health. Most importantly, it assesses the health of the kidneys, but it also indicates the presence or absence of a urinary tract infection. The urine may be collected by catching some in a cup or by performing a cystocentesis that removes urine from the bladder by means of a needle passed through the belly into the urinary bladder.
• Biopsy of the tumor. This is an essential procedure for definitive diagnosis of bone cancer. Because it is painful to biopsy bone, this procedure is done under anesthesia. Your veterinarian or a veterinary specialist will insert a large needle into the affected area of bone to remove a core of tissue. The tissue sample is then submitted to a pathologist to determine the form/type of cancer. There is a minimal risk of fracturing the bone with this procedure, but because it is essential for the diagnosis, the risk is worth taking. Your pet is likely to be in pain after this procedure and pain medications are often administered. A narcotic patch placed on the skin to allow absorption of a constant level of pain reliever may also be used.

Your veterinarian may recommend additional diagnostic tests to ensure optimal medical care. These are selected on a case-by-case basis. An example includes:

• Bone scan. This is a specialized type of X-ray that is done to look for spread of the cancer to other bones. These scans are available only in limited veterinary referral centers, because they require specialized machines. To perform this type of scan, your pet is given an injection of a radioactive substance that localizes to sites of bone damage. A special camera is then used to take X-rays of all the bones in the body. It is a non-specific test that requires interpretation by a radiologist, but it can help determine if there are multiple bones that are affected by bone cancer. This is a very uncommon finding but one that would drastically influence the treatment recommendations. There is no risk to your pet from this type of radiation, but he/she will usually be required to stay at least one night in a special holding cage in the hospital. This allows the radioactive substance to be expelled from your pet’s body via urine and feces.

In-depth Information on Treatment

Treatments for osteosarcoma may include the following:

• Surgery. The most common approach to the treatment of osteosarcoma involves removal of the tumor en bloc, which means bone along with normal surrounding tissues. Because osteosarcoma most commonly affects the limbs, amputation of the affected limb is usually the primary treatment. This is the best means of removing the cancer and it will typically provide relief from pain caused by the cancer. However, it is truly only palliative (pain relieving), because survival is not enhanced by performing amputation alone. Survival with amputation alone in dogs is in the range of four to six months on average.
  Amputation may seem like a drastic measure, and you may question your dog’s ability to deal with such a surgery. However, most animals do very well on three legs and the degree of pain associated with the surgery is minimal compared to the pain that they have experienced with the tumor. There is a period of recovery necessary after a limb amputation. Most animals spend at least one postoperative day in the hospital. Narcotic pain medications and fluid supportive care are often given. After your pet is released from the hospital, you will need to restrict activity severely until the surgical site heals and the sutures/staples are removed, usually 10 to 14 days after surgery. During this time your pet should not jump, play or climb stairs unattended. You will also need to keep the surgical site clean and dry.
• Chemotherapy. Because of the highly metastatic nature of osteosarcoma, amputation alone is not sufficient to prolong survival substantially. Chemotherapy is usually prescribed in the post-operative period once healing has occurred and the sutures/staples have been removed. These drugs are given as intravenous injections in the leg, and typically follow a set schedule of being given every three weeks for a total of four to six doses. Many different types of chemotherapy drugs are available, and your veterinarian may wish to refer you to a veterinary oncologist in your area who can advise you on treatment options. The most commonly utilized drugs are cisplatin, carboplatin and adriamycin.

Every oncologist has a preference and may pick certain drugs or combinations based on your pet’s general health and the appearance of the tumor. All of these treatments have about the same impact on prolonging survival, an average of about 10 to 12 months from diagnosis. Common side effects include a decrease in your pet’s white blood cell count approximately seven to ten days after each treatment, nausea, vomiting and/or diarrhea. Each of these drugs also has unique side effects, which your oncologist will discuss with you, depending on the drug chosen for your pet.

• Radiation therapy. This is a type of treatment in which a beam of radiation is directed at the tumor to kill the cells, and usually results in pain relief. It is a highly specialized treatment and is available only in some referral veterinary centers for select cases. It may be worth considering if your dog has existing conditions that disqualify him for amputation or if there is already evidence of metastasis. It typically involves multiple treatments given over a three-week period. Though it often results in pain relief, it seldom prolongs survival substantially, and the average survival time is still six months. Chemotherapy can be administered in conjunction with the radiation and may serve to control metastatic disease. Your dog will usually not experience any adverse side effects from this treatment, unlike what is typically associated with radiation therapy in people. The oncologist or radiation oncologist prescribing these treatments will discuss further details with you.
• Pain medication. If you choose not to pursue any of the above treatments, the administration of both narcotic and non-narcotic anti-inflammatory medication can help in making your dog more comfortable for a period of time.
• Limb-sparing surgery. This is a highly-specialized type of surgery where the bone containing the tumor is removed and replaced by a donor bone. This surgery is technically demanding and is available only at a limited number of veterinary surgical referral centers where there are surgeons trained to perform such surgeries and where a bone bank is available. It is most useful for very large- or giant-breed dogs whose ability to walk after an amputation may be questionable. If you feel your pet may be a candidate, the oncologist in your area can inform you of the pros and cons of this procedure and provide you the names of the centers that perform this procedure.