WHAT IS OSTEOSARCOMA?
Osteosarcoma is by far the most common bone tumor of the dog, usually striking the leg bones of larger breeds. Osteosarcoma usually arises in middle aged or elderly dogs but can arise in a dog of any age with larger breeds tending to develop tumors at younger ages.
Osteosarcoma can develop in any bone but the limbs account for 75-85% of affected bones. Osteosarcoma of the limbs is called “appendicular osteosarcoma.”
Osteosarcoma develops deep within the bone and becomes progressively more painful as it grows outward and the bone is destroyed from the inside out. The lameness goes from intermittent to constant over 1-3 months. Obvious swelling becomes evident as the tumor grows and normal bone is replaced by tumorous bone.
Tumorous bone is not as strong as normal bone and can break with minor injury. This type of broken bone is called a “pathologic fracture” and may be the finding that confirms the diagnosis of bone tumor. Pathologic fractures will not heal and there is no point in putting on casts or attempting surgical stabilization.
RADIOGRAPHS: One of the first steps in evaluating a persistent lameness is radiography (x-rays). Bone tumors are tender so it is usually clear what part of the limb should be radiographed. The osteosarcoma is creates some characteristic findings:
- The “lytic lesion” – looks like an area of bone has been eaten away.
- The “sunburst” pattern – shows as a corona effect as the tumor grows outward and pushes the more normal outer bone up and away.
- A pathologic fracture may be seen through the abnormal bone.
- Osteosarcoma does not cross the joint space to affect other bones comprising the joint.
In most cases, radiography is all that is needed to make the diagnosis but sometimes there are ambiguities.
BIOPSY: A tiny section of bone can be removed for laboratory analysis. This type of analysis is considered to be absolute proof of diagnosis. The procedure is associated with some pain and our local oncologists have suggested that biopsy is not needed if the radiographs show an obvious bone tumor. If there is any question about the lesion on the radiographs, a bone biopsy should provide clear results.
Sometimes a bone tumor is surrounded by an area of bone inflammation and it may be difficult to get a diagnostic sample and several samples must be taken. These samples are too small to cause a pathologic fracture.
Amputation of the affected bone is recommended for any tumor involving bone. When the malignant structure has been removed, it is submitted for biopsy and the diagnosis confirmed at that time. Biopsy before amputation is felt to simply add a painful procedure to the patient and, if possible, is reserved for tissue already amputated.
WHAT IF IT ISN´T REALLY AN OSTEOSARCOMA?
The location and radiographic appearance of the osteosarcoma in the limb are quite classic but there are a few outside possibilities that should at least be mentioned. There are only a few other possible conditions that cause lytic lesions in bone: the chondrosarcoma, or the squamous cell carcinoma, or the synovial cell sarcoma.
CHONDROSARCOMA: Chondrosarcoma is a cartilage tumor, possibly not as malignant as the osteosarcoma. The chondrosarcoma generally occurs on flat bones such ribs or skull bones and is not usually found in the limbs. Still, should a chondrosarcoma occur in the limb, treatment recommendations would still include amputation of the affected bone and biopsy of the tissue after amputation would allow for any adjustments in chemotherapy.
SQUAMOUS CELL CARCINOMA: The squamous cell carcinoma is a tumor of the external coating of the bone (called the “periosteum”). This is a very destructive tumor locally but it tends to spread relatively slowly. Again, a bone suspected of malignant tumor should be amputated and the tissue then analyzed and treatment adjustments made thereafter. The squamous cell carcinoma tends not to arise in the same bone areas as the osteosarcoma; it tends to arise in the jaw bones or in the toe bones.
SYNOVIAL CELL SARCOMA: This is a tumor of the joint capsule lining. It´s hallmark is that it affects both bones of the joint. The osteosarcoma, no matter how large or destructive it becomes, will never cross over to an adjacent bone.
FUNGAL BONE INFECTION: Coccidiodes immitis is a fungus native to the Lower Sonoran Life Zone of the South West U.S. It is the infectious agent of the disease called “San Joaquin Valley Fever” or just plain “Valley Fever.” (More scientifically, the condition is called “coccidiodomycosis.”) In most cases, infection is limited to a few calcified lymph nodes in the chest and possibly lung disease. In some rare cases, though, the fungus disseminated through the body and can cause a very proliferative bone infection. The bone infection of coccidiodomycosis is proliferative and lacks the lytic lesions that are so typical of the osteosarcoma.
The bottom line is that bone malignancy should be treated with amputation followed by adjunctive treatment. What the adjunctive treatment is, depends on what the bone tumor is.
TREATMENT OF OSTEOSARCOMA INVOLVES TWO ASPECTS:
- TREATING THE PAIN CAUSED BY THE BONE TUMOR
- FIGHTING THE SPREAD OF THE CANCER.
HOW DO WE TREAT THE PAIN?
Keep in mind that dogs are usually euthanized due the pain in the affected bone. Treating the pain successfully will allow a dog to live comfortably.
AMPUTATION OF THE LIMB: Removal of the affected limb resolves the pain in 100% of cases. Unfortunately, many people are reluctant to have this procedure performed due to misconceptions.
While losing a leg is very handicapping to a human (who only has two legs total), losing one leg out of four does not restrict a dog´s activity level. Running and playing are not inhibited by amputation (after the surgical recovery period is over).
While losing a limb is disfiguring to a human and has social ramifications, dogs really are not so self-conscious about their image. The dog will not feel disfigured by the surgery; it is his or her owner that will need to adjust to the new appearance of the dog.
Median survival time for dogs who do not receive chemotherapy for osteosarcoma is 4-5 months from the time of diagnosis regardless of whether or not they have amputation. Do you want your dog´s last 4-5 months to be painful or comfortable?
LIMB SPARING SURGERY: Limb-sparing techniques developed for humans have been adapted for dogs. To spare the limb (and thus avoid amputation), the tumorous bone is removed and either replaced by a bone graft from a bone bank or the remaining bone can be re-grown via a new technique called “bone transport osteogenesis.” The joint nearest the tumor is fused (ie fixed in one position and cannot be flexed or extended.)
Limb sparing cannot be done if more than 50% of the bone is involved by tumor or if neighboring muscle is involved.
Limb sparing does not work well for hind legs or tumors of the humerus (“arm” bone.)
Limb sparing works best for tumors of the distal radius (“forearm” bone).
Complications of limb sparing can include: Bone infection, implant failure, tumor recurrence, and fracture.
RADIOTHERAPY FOR PAIN CONTROL : Radiation doses can be applied to the tumor in 3
doses (the first two doses 1 week apart, the second two doses 2 weeks apart.) Improved limb function is usually evident within the first 3 weeks and typically lasts 4 months. (Our local oncologists report a range of 0-19 months.) When pain returns, radiation can be re-administered for further pain relief if deemed appropriate based on the stage of the cancer at that time. In the Los Angeles area, a course of 3 doses of radiotherapy typically costs $700-$800.)
When pain is relieved in the tumorous limb, there is an increase in activity which can in turn lead to a pathologic fracture of the bone.
Radiotherapy does not produce a helpful response in about 1/3 of patients. (Remember, amputation controls pain in 100% of cases but if amputation is simply not an option, there is a 2 out of 3 chance that radiotherapy will control the pain.)
DRUGS: Analgesic medications such as carprofen, etodolac, aspirin, butorphanol, and fentanyl patches are all available but, unfortunately, they are no match for the pain involved in what amounts to a slowly exploding bone. These medications may be palliative at some stage but generally do not provide meaningful pain relief long term.
HOW DO WE TREAT THE CANCER?
Osteosarcoma is unfortunately a fast spreading tumor. By the time the tumor is found in the limb, it is considered to have already spread. (Osteosarcoma spreads to the lung in a malignant process called “metastasis.” Prognosis is substantially worse if the tumor spread is actually visible on radiographs in the chest so if chemotherapy is being contemplated, it is important to have chest radiographs taken.
Chemotherapy is the only meaningful way to alter the course of this cancer.
Young dogs with osteosarcoma tend to have shorter survival times and more aggressive disease than older dogs with osteosarcoma.
Elevations of “Alkaline phosphatase,” one of the enzymes screened on a basic blood panel, bode poorly. These dogs have approximately 50% of the survival times quoted below for each protocol.
CISPLATIN (given IV every 3-4 weeks for 3 treatments)
The median survival time with this therapy is 400 days.
Survival at one year: 30-60% (depending on what??)
Survival at two years: 7-21%
Giving less than 3 doses does not increase survival time (ie if one can only afford one or two treatments, it is not worth the expense of therapy)
Cisplatin can be toxic to the kidneys and should not be used in animals with pre-existing kidney disease.
CARBOPLATIN (given IV every 3-4 weeks for 4 treatments)
Similar statistics to cisplatin but carboplatin is not toxic to the kidneys and can be used if the patient has pre-existing kidney disease. Carboplatin is substantially more expensive than cisplatin.
DOXORUBICIN (given IV every 2 weeks for 5 treatments)
The median survival time is 365 days.
10% still alive at two years.
Toxic to the heart. An ultrasound examination is needed prior to using this drug as it should not be given to patients with reduced heart contracting ability.)
DOXORUBICIN AND CISPLATIN IN COMBINATION (both given IV together every 3 weeks for 4 treatments)
48% survival at one year
30% survival at two years
16% survival at three years.
WHAT EXACTLY IS “MEDIAN” SURVIVAL TIME
When a population is evaluated statistically, there are a number of ways the central tendency of the group can be evaluated. The “median” is the value at which 50% of the group falls above and 50% of the group falls below. This is a little different from the “average” of the group, though more people are familiar with this term. When one evaluates “median survival times” one is looking at a 50% chance of surviving longer than the median (and a 50% chance of surviving less than the median).
WHAT EXACTLY DOES CHEMOTHERAPY PUT MY DOG THROUGH?
Most people have an image of “the chemotherapy patient” either through experience or the media and this image typically includes lots of weakness, nausea, and hair loss. In fact, the animal experience in chemotherapy is not nearly as dramatic. After the pet has a treatment, one should expect 1-2 days of lethargy and nausea. This is often substantially palliated with medications like Zofran® (a strong antinauseal commonly used in chemotherapy patients). These side effects are worse if a combination of drugs is used but the pet is typically back to normal by the third day after treatment. Effectively, you are trading 8 days of sickness for 6-12 months of quality life. Hair loss is not a feature of animal chemotherapy.
AXIAL OSTEOSARCOMA
While osteosarcoma of the limbs is the classical form of this disease, as mentioned, osteosarcoma can develop anywhere there is bone. “Axial” osteosarcoma is the term for osteosarcoma originating in bones other than limb bones, with the most common affected bones being the jaws (both lower and upper). Victims of the axial form of osteosarcoma tend to be smaller, middle-aged, and females outnumber males 2:1.
In the axial skeleton the tumor does not grow rapidly as do the appendicular tumors thus leading to a more insidious course of disease. The tumor may be present for as long as two years before it is formally diagnosed. An exception is osteosarcoma of the rib, which tends to be more aggressive than other axial osteosarcomas.
Treatment for axial osteosarcoma is similar to that for the appendicular form: surgery followed by chemotherapy. There is one exception, that being osteosarcoma of the lower jaw. Because of the slower growth of the axial tumor and the ability to remove part or all of the jaw bone with little loss of function or cosmetic disfigurement, it has been reported that 71% of cases survived one year or longer with no chemotherapy at all.
For more details on treatment and expectations, we encourage our clients to see a veterinary oncology specialist. Do not consider the internet as a replacement for the specifics you can receive in a one-on-one consultation with an expert.
Please visit the veterinary locator site at http://www.vetquest.com