Osteosarcoma is a form of bone cancer in dogs that accounts for 85% of primary bone tumors diagnosed, making it the most common bone tumor. It is a highly aggressive tumor, characterized by local invasion and destruction of the bone as well as early metastasis (spread to other organs, the most common site of metastasis being the lungs). Osteosarcoma most commonly affects the limbs (or the appendicular skeleton) of large to giant breed dogs. It can also occur in other bones such as the skull, ribs, vertebrae, and pelvis (the axial skeleton) which are more common sites in smaller dogs.
Clinical Signs of Osteosarcoma
The signs associated with bone cancer in dogs may be nonspecific and depend on the location. Tumors involving the limbs often cause various degrees of lameness, pain, and muscle wasting, and a firm swelling may become evident as the tumor grows. The pain can cause other problems such as irritability, aggression, loss of appetite, weight loss, whimpering, restlessness or reluctance to exercise.
Diagnosis and Work-up (Staging)
The initial evaluation for any dog suspected of having a bone tumor will include a complete physical exam, blood work (complete blood count, serum chemistry profile), urinalysis, radiographs of the primary site as well as the lungs, and an aspirate or biopsy of the lesion. In some cases a CT scan is recommended to evaluate for metastasis and/or surgical planning. Unfortunately, 80-90% of dogs diagnosed with osteosarcoma already have microscopic spread to the lungs but we are not able to detect these microscopic lesions with X-rays or a CT scan. The work-up and staging are important for two reasons. First, it is necessary to determine the tumor type and extent of the cancer to determine the appropriate treatment and prognosis. Secondly, it also provides the clinician with information regarding your dog’s general health and may identify concurrent medical or musculoskeletal problems all of which may influence the treatment recommendations.
Treatment Options for Osteosarcoma in Dogs
Treatment recommendations for bone tumors depend on multiple factors and the ultimate goal is to help improve your pet’s quality of life. Treatment can be divided into two parts:
- Pain management/treatment of the primary tumor
- Treatment to address the risk for metastatic disease
The initial focus of treatment is pain management. Osteosarcoma causes pain due to the invasion and destruction of normal bone by the tumor. There are several options for pain management depending on the location of the tumor. These include surgery (amputation for osteosarcoma of the limb), oral analgesics (pain relievers), radiation therapy and/or intravenous bisphosphonate therapy.
For osteosarcoma involving a limb, amputation is the best method for pain control. It is much easier to control post-operative pain than it is to control chronic bone pain. An amputation also eliminates the risk of a fracture at the tumor site. Fractures occur because the tumor weakens the bone to the point that it can break. Fortunately, the risk of a fracture is low (~20%). Most dogs will do very well with an amputation and often resume many of their normal activities such as running, playing, swimming, etc. It is important to understand that if an amputation is performed for osteosarcoma without chemotherapy, most patients develop metastatic disease and succumb to the cancer within 4-6 months, which is why we don’t always recommend amputation as a sole therapy.
For dogs that are not candidates for an amputation or situations in which a caretaker isn’t interested in amputation, we have several options for pain management that can be used singly or in combination. There is no “one size fits all” when it comes to pain management and we may need to try several combinations until we find what works best for a particular dog.
Oral Analgesics include non-steroidal anti-inflammatory drugs (NSAIDS) (or aspirin-like drugs), steroids and narcotics. It is common to combine a NSAID with a pain medication like gabapentin, amantadine, codeine and fentanyl.
Palliative Radiation Therapy (PRT) involves the use of several large doses of radiation administered once a week, for 3-4 doses. PRT appears to be the best option for pain management if surgery (amputation) is not pursued. The goal of this form of radiation is to improve/maintain the patient’s quality of life with minimal negative impact (minimal side effects). Approximately 70% of dogs experience pain relief with PRT which lasts for an average of 4 months. It may be possible to repeat a course of PRT, depending on the dog, though the duration of the response is usually shorter than the original response. PRT for bone tumors (appendicular) is for pain management and is not a treatment for the primary tumor. In most patients the tumor continues to progress even though the patient is more comfortable and fracture is still a risk.
Bisphosphonates: Bisphosphonatesare a class of drugs that are meant to slow or prevent bone breakdown as well as increase bone production. This class of drugs is used frequently in women to prevent osteoporosis (ie. Boniva or Fosamax). Several of the drugs have also been used to help control pain from bone destruction. The most commonly used drug is zoledronate. This medication is administered intravenously once every 4 weeks as a 15 minute infusion. Initially we plan 2-3 treatments and then determine if there has been an improvement. If there has been an improvement, this drug can be repeated for as long as there is response and the patient tolerates the drug. Side effects are rare, but we do monitor kidney function prior to each treatment due to a low risk for kidney damage.
The most common cause of death in dogs with appendicular osteosarcoma, especially those undergoing an amputation, is metastasis to the lungs. Because of this, systemic chemotherapy is recommended as a follow-up to surgery for dogs with appendicular osteosarcoma. Chemotherapy is not likely to cure most dogs with osteosarcoma but can significantly prolong their quality of life to 10-12 months on average with 20% of patients alive at 2 years. Survival time for dogs treated with an amputation alone is about four months. We currently recommend the chemotherapy drug Carboplatin. This chemotherapy agent is administered intravenously once every three weeks for 6 doses. Most dogs tolerate chemotherapy well with less than 10% experiencing side effects significant enough to warrant an outpatient visit and less than 1% will require hospitalization or experience any life-threatening side effects secondary to chemotherapy.
The goal of any treatment is to maintain your pet’s quality of life. Your oncologist will work with you to determine the best course of treatment for you and your dog.