Dr. Rachel Mar is here to educate and inform pet parents about Osteosarcoma. Continue reading to learn more about what Dr. Rachel has to say about this bone cancer that impacts large and giant breed dogs.
Osteosarcoma is by far the most common bone tumor of dogs, commonly found in the leg bones of larger breeds. This bone tumor usually occurs in middle aged or elderly dogs but can be diagnosed in a dog of any age - larger breeds tend to develop tumors at younger ages.
Osteosarcoma can develop in any bone but the limbs account for 75% to 85% of affected bones. Osteosarcoma of the limbs is called appendicular osteosarcoma. It 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 may start intermittently then increases to a constant, worsening lameness over the course of weeks to 2-3 months. An obvious swelling over the affected region of the leg becomes evident as the tumor grows as normal bone is being replaced by cancerous bone.
Since cancerous bone is not as strong as normal bone, it 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.
Image From Whole Dog Journal
It is best to have your dog seen by a veterinarian if any lameness is present, especially if a swelling is seen over the affected leg.
How is Osteosarcoma Diagnosed?
One of the main diagnostics your veterinarian will suggest after examining your dog for any lameness are radiographs (x-rays). This is the one of the best ways to confirm if your dog has osteosarcoma. With osteosarcoma, the lytic lesion looks like an area of bone has been eaten away on x-ray. Bone tumors are tender so it is usually clear what part of the limb should be radiographed. The osteosarcoma creates some characteristic findings.
- The sunburst pattern. This illustrates as the name implies - as the tumor grows outward, it pushes the more normal outer bone up and away.
- As mentioned before, a pathologic fracture may be seen through the abnormal bone.
- Osteosarcoma does not cross the joint space to affect other bones in the joint.
Radiography is almost completely diagnostic in most cases, but there are a few other less common conditions that can mimic the appearance of a bone tumor, so your veterinarian may want to pursue further testing. If a basic blood panel and urinalysis haven't been done, this would be a good addition to your pet’s work up so that your veterinarian has a better idea regarding their internal health, such as liver and kidney function. Elevations of alkaline phosphatase, one of the enzymes screened on a basic blood panel, negatively affect a dog’s prognosis. These dogs have approximately 50% of the survival times quoted below for each protocol.
The most definitive and ‘gold standard’ diagnostic tool to confirm the presence of osteosarcoma is tissue sampling through biopsy and needle aspirate. Due to the important decisions which would be make on the diagnosis of a bone tumor, it is best to obtain a tissue sample for confirmation.
A small piece of bone can be harvested surgically. The bone is preserved, sectioned, and examined under the microscope to confirm the diagnosis of osteosarcoma. There are several problems associated with this diagnostic. Sometimes a bone tumor is surrounded by an area of bone inflammation and it may be difficult to get a representative sample. The tiny hole that results when a core of bone is removed can create a weak spot and the bone can actually break. Even if the procedure goes well, often there is increased pain and lameness for the patient afterwards. With so many potential problems, most specialists have switched to needle aspirate for diagnosis.
With needle aspirate, a large bore needle is inserted into the area of the tumor and cells are withdrawn for analysis. A full core of bone is not removed, just a sampling of cells. This is usually sufficient to confirm osteosarcoma. If there is ambiguity, certain stains can often settle any questions the pathologist may have.
What are the treatment options?
Osteosarcoma is unfortunately a fast-spreading tumor. By the time the tumor is found, it is considered to have already spread. That being said, prognosis and treatment options depend on whether or not the tumor spread has progressed. Osteosarcoma spreads to the lung in a malignant process called metastasis. Prognosis is substantially worse if the tumor spread is visible on chest radiographs, so if you are contemplating chemotherapy, chest radiographs should be taken. Some specialists recommend nuclear imaging of the skeleton to identify any spread to other bones, which might also alter recommendations; however, this form of imaging is not readily available.
Keep in mind that dogs are usually euthanized because of the pain in the affected bone. Treating the pain successfully will allow a dog to live comfortably and extend life expectancy by virtue of extending comfort. There are two ways to address the pain: amputating the limb and palliative radiotherapy (usually combined with periodic bisphosphonate infusion treatments).
For most patients, there is one tumor on a leg and no visible tumor spread in the lungs. These are the patients with the best potential results and they are good candidates for amputation. Patients with a lot of arthritis in the other legs or with tumor spread evident in the chest already are probably not candidates for amputation and it may be more appropriate to keep the leg and simply relieve the pain with radiotherapy.
Since the tumor in the limb is the source of pain, it makes sense that amputating the limb would resolve the pain.Removing the affected limb resolves the pain in 100% of cases. Unfortunately, many people are reluctant to have this procedure performed because of misconceptions.
While losing a leg is handicapping to a human, losing one leg out of four does not restrict a dog's activity level. Running and playing are not inhibited by amputation after recovery from surgery. That said, if the remaining legs are arthritic, the stress on them can pose a mobility issue. While losing a limb is disfiguring to a human and has social ramifications, dogs are not self-conscious about their image. The dog will not feel disfigured by the surgery; it is the owner that will need to adjust to the dog’s new appearance.
Median survival time for dogs who do not receive chemotherapy for osteosarcoma is 3 to 5 months from the time of diagnosis regardless of whether or not they have amputation. If a tumor is found in the lymph nodes local to the leg being amputated, this gives a poorer prognosis.
Limb-sparing Surgery (removing the tumor but not the leg)
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 (i.e. fixed in one position and cannot be flexed or extended.)
Generally, limb-sparing is not a viable option as the cancer is so aggressive and there are some cons to this procedure. Limb sparing cannot be done if more than 50% of the bone is involved by tumor or if neighboring muscle is involved. It 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.
While amputation can be viewed as a pain management strategy, limb-sparing is only performed in conjunction with chemotherapy. It is important to keep in mind that grafting of a new bone structure requires healing time and that a great deal of post-operative confinement time is needed (in a patient whose life expectancy is going to be measured in months). For the right patient, limb-sparing can be the best choice but be sure to understand all the details of post-operative care from the specialist.
Palliative Radiotherapy for Pain Control
Sometimes amputation is simply not the right choice. Luckily, there is an effective alternative treatment. Radiation can be applied to the tumor in two, three or four doses, depending on the protocol. Improved limb function is usually evident within the first 3 weeks and typically lasts 2 to 4 months. When pain returns, radiation can be given again for further pain relief if deemed appropriate based on the stage of the cancer at that time.
There are a couple of caveats:
- When pain is relieved in the tumorous limb, there is an increase in activity that can in turn lead to a pathologic fracture of the bone.
- Radiotherapy does not produce a helpful response in about 1/4 of patients. Remember, amputation controls pain in 100 percent of cases but if amputation is simply not an option, there is a 3/4 chance that radiotherapy will control the pain.
- Current standard treatment involves pairing palliative radiation with monthly infusions of medications called bisphosphonates.
This class of drug has become the standard of care in humans with bone tumors and have been found helpful in managing osteosarcoma pain in dogs as well. Bisphosphonates act by inhibiting bone destruction, which in turn helps control the pain and bone damage caused by the bone tumor. The most common bisphosphonate in use for dogs has been pamidronate, though a new drug zoledronatehas recently been developed. Treatment is given as an IV drip over two hours in the hospital every 3 to 4 weeks. In humans, an assortment of potential side effects have emerged, but so far these issues have not been problems for dogs and cats. Bisphosphonates are important in managing bone tumor pain in patients that have not undergone amputation.
At this time there are numerous pain medications available for dogs with osteosarcoma. No single medication, however, is a match for the pain involved in what amounts to a slowly exploding bone. A combination of medications is needed to be reasonably palliative and should be considered only as a last resort if amputation or radiation therapy will not be pursued. There are several types of drugs that can be combined.
Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)
These are anti-inflammatory pain relievers developed for dogs: carprofen, etodolac, deracoxib, meloxicam, firocoxib, and others. These are typically given once or twice daily in tablet form at home. The patient should have good liver and kidney function in order to take medications of this class.
Narcotic Pain Relievers
While these drugs do not have anti-inflammatory properties, they are well-known analgesics and have been used in an assortment of forms for thousands of years. They are particularly useful in chronic pain because they do not interact negatively with other pain relievers. Drowsiness is a potential side effect.
Gabapentin & Amantadine
Gabapentin works on neurologic pain and is rapidly surfacing in the treatment of arthritis, surgical pain, and other chronic pain states. Amantadine works by reducing what is called wind up, a phenomenon where nerves become sensitized to pain leading to the experience of pain from stimuli that normally do not cause pain.
These different drugs are often given together to create analgesia to the osteosarcoma patient when amputation and radiotherapy are not going to happen. It is important to realize that there is a limit to how much pain relief can be achieved against a bone tumor with only pills. It will not be long before the pain of this tumor, as evidenced by not using the leg, tenderness to the touch, etc., overpowers the effect of oral medications.
Chemotherapy is the most useful 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.
Cisplatin (given by IV every 3 to 4 weeks for 3 treatments)
The median survival time with this therapy is 400 days. Giving less than 3 doses does not increase survival time (i.e. 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 by IV every 3 to 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% of patients are still alive at 2 years. This drug is sadly 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 four treatments)
48% survival at 1 year, 30% survival at 2 years, 16% survival at 3 years.
What Does Chemotherapy Put My Dog Through?
The image of a chemotherapy patient is that in which one experiences lots of weakness, nausea, and hair loss. However, the animal experience in chemotherapy is not nearly as dramatic. After the pet has a treatment, expect 1 to 2 days of lethargy and nausea. This is often substantially helped with medications like Zofran, a strong anti-nausea drug 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 to 12 months of quality life. Hair loss is not usually a feature of animal chemotherapy. In dogs, hair loss may occur in breeds that have continuously growing coats, such as poodles, Scottish terriers, and Westies.
While osteosarcoma of the limbs is the classical form of this disease, 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 female.
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 there 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.
I’ve unfortunately lost two dogs to osteosarcoma. Both were Rottweilers who had the appendicular form. Therefore, I can attest to the rapid progression and aggressive nature of this cancer. Please make an appointment with your veterinarian if your dog shows any signs of lameness, and definitely so if your dog is painful to the touch or has a swelling on the affected limb. The best prognosis of this cancer occurs when immediate action is taken.
We thank Dr. Rachel Mar so much for her time to educate on this important topic.
For more information on Dr. Rachel Mar, be sure to visit her website.
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