Downeast Dog News
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NO BONES ABOUT IT

Canine Osteosarcoma
By Gail Mason, DVM, MA, DACVIM | Oct 01, 2020

Osteosarcoma (OSA) is the most common primary bone tumor in dogs. Large and giant breed dogs have the highest risk of this malignancy. This tumor causes destruction of the bone at the site of origin, and worse, it readily travels (metastasizes) to distant sites such as the lungs. It is one of the few cancers in dogs that causes considerable pain in its active stages. Osteosarcoma is an aggressive cancer that arises most commonly in the limbs, referred to as appendicular OSA. Osteosarcoma can erode the bone structure to the point of causing a spontaneous (pathological) fracture of the affected limb.

The disease usually manifests as a lameness which usually becomes significant within 2-3 months. An obvious swelling occurs at some point, which is often warm to the touch and is painful to the dog.

Any dog that has progressive lameness without known cause, should be screened with radiographs (x-rays) of the affected area. In most cases, OSA results in a very characteristic “sunburst” pattern as the tumor tissue within the bone expands and pushes the normal bone outward. This can be accompanied by fractures. Another telltale feature is that the tumor does not “cross the joint” to the next bone. There are rare instances of chronic bone infections (osteomyelitis) from bacteria or fungal organisms which can mimic OSA. If the radiographic evidence is not convincing for a diagnosis of OSA, further diagnostics such as a needle aspirate or needle aspirate of the affected area can readily be obtained.

If a diagnosis of OSA is confirmed, your veterinarian may refer you to a specialist to explore options for treatment. Since OSA spreads microscopically before the diagnosis is even known, it is best to “stage” the patient to look for presence of detectable metastatic disease. Lymph nodes local to the site of the tumor are examined as well as either a full set of thoracic (chest) radiographs or a CAT scan of the lungs.

Chronic pain is the most significant feature of bone cancer as well as an increasing risk for a sudden fracture. Treatment options are aimed at extending the dog’s good quality life and controlling pain as currently there is no cure for this disease. The most effective treatment is to remove the affected limb (if there is no evidence of tumor spread). While often disconcerting to owners, the pain is eliminated almost immediately. Veterinarians do not take this decision lightly, and it should be discussed thoroughly. Fortunately, our canine friends do not have the mental or social ramifications of feeling “handicapped,” and are not restricted in any way from normal activities. Being the incredibly adaptive creatures that they are, patients are usually up and ambulatory within hours of surgery. Surgical intervention, alone, will eliminate the dog’s pain and generally provides 3-5 months of excellent quality of life.

The addition of chemotherapy is currently the only way to significantly delay the course of OSA. It is only effective when the primary tumor has been surgically removed. Carboplatin is the most recommended therapeutic treatment for OSA patients. It is administered intravenously, as an outpatient once every three weeks for a total of 4-6 treatments. Though this drug is a human anti-cancer drug, it is well tolerated in dogs. Mild lethargy for 1-2 days after treatment is the most common reported symptom. Fortunately, dramatic side effects that we are all too familiar with including nausea, vomiting, diarrhea, fever, weight loss and hair loss are very uncommon in dogs. Dogs who complete their therapy reportedly have a 35-60% 1-year pain-free survival rate and a 10-21% two- year pain free survival rate. Eventually, the disease spreads to the lungs which ultimately leads to demise of the patient.

If surgery is not the chosen route, what else can help? Pain control is the mainstay of palliative treatment for this disease. The outpatient administration of “bone strengthening” drugs such as bisphosphonates (approximately every 3-4 weeks), can be extremely rewarding in dogs with OSA. These are not chemotherapy agents, but they slow down bone destruction by the tumor, preserve bone strength, and reduce bone pain. Side effects are very uncommon, and most owners note significant improvement. Patients can be successfully managed over months in this way. This can be combined with outpatient radiation therapy (once weekly for three treatments) to further control pain and swelling. Dogs do not suffer any “radiation sickness” effects which makes this a popular choice among clients. Pain can also be controlled with medications such as “nonsteroidal anti-inflammatory drugs” (NSAIDS), along with narcotic analgesics if deemed appropriate.

While this disease remains a formidable foe in both human and veterinary medicine, hope is on the horizon. New therapeutic trials with targeted immunotherapy drugs, enzyme pathway inhibitors and anti-tumor vaccines are underway. Lastly, it should be noted that recent research has shown that delaying spaying and neutering of dogs of high OSA risk breeds until sexual maturity has a sparing effect on the development of OSA. This would be 12-15 months for males, and after 1-2 estrus cycles in females. Ironically, this delay increases the risk for mammary cancer in female dogs, but the latter cancer can be detected, and treated earlier with excellent outcomes. OSA, our “Dogs in The Fight” are coming for you!!!