What is Osteosarcoma in Pets?
Osteosarcoma (OSA) is also known as osteogenic sarcoma and is a member of the family of tumors known as primary bone tumors. Primary means that the tumor started primarily in the bone and does not represent spread (metastasis) of cancer from some other site in the body.
Many tumors in other parts of the body will spread to bone, and therefore it is crucial to differentiate primary bone tumors from metastatic disease. Osteosarcoma is the most common primary bone tumor diagnosed in dogs. In fact, when a tumor is found in the bone, it is ultimately diagnosed as OSA more than 90% of the time.
Osteosarcoma tends to occur in large and giant breeds of dogs and also tends to occur in major weight bearing bones. Typically, we diagnose OSA in older patients, but we also see a fairly high incidence in younger dogs as well. This cancer is characterized by a high rate of spread to other parts of the body, especially the lungs and other bones. In contrast, this disease is rare in cats and doesn't appear to be metastatic.
What are the symptoms of OSA?
Lameness and swelling in the affected limb are the most common symptoms seen. Oftentimes, the lameness is initially believed to be due to some traumatic episode such as jumping out of a car or off of a bed. When a lameness doesn’t resolve as would be expected with trauma, radiographs are recommended.
How is the diagnosis made?
If a radiograph is suspicious of a primary bone tumor, a biopsy is necessary to make a definitive diagnosis. Most primary bone tumors have a characteristic appearance on radiographs. The bone often appears to have been “eaten away” by the tumor. Sometimes the radiographic lesion may not have a classic appearance.
A bone biopsy is typically done under a general anesthesia for the comfort of the patient. A tiny incision is made in the skin above the suspected tumor. A small core of bone is taken using a “Jamshidi” bone biopsy instrument. The sample taken is then sent to the pathology laboratory for analysis. In cases where patients are extremely painful or have suffered pathologic fractures, it may not be in the best interest of the patient to wait for biopsy results.
At ACIC, we take a pre-operative bone aspirate while the patient is being prepped for surgery. We read the cytology on site and if the sample is consistent with cancer, we proceed with amputation. Cytology can rule out fungal disease or bacterial infection but cannot always distinguish between the type of sarcoma present (i.e. osteosarcoma vs. chondrosarcoma).
Staging the disease:
It is extremely important to determine if the cancer has spread (metastasized). OSA can go to any tissue in the body, but tends to spread to lungs and other bones. Up to 10% of patients will have visible cancer in the lungs at the time of diagnosis.
A complete work-up includes:
· Complete blood count (CBC), serum chemistry panel, urinalysis
· Thoracic radiographs (3 views)
· Radiographic survey of appendicular skeleton
· Bone biopsy (or pre-operative bone aspirate)
· CT scan if available
Can my pet still be treated if the cancer has spread?
It is generally not recommended to pursue definitive treatment once the cancer has spread beyond the primary site. Treatment at this stage would be directed towards palliation (relieving pain) and could include radiation therapy to the tumor site (2-4 dosages of radiation), chemotherapy, or bisphosphonate therapy.
Treatment of osteosarcoma:
In cats, amputation alone is often curative. In dogs, surgery and chemotherapy are the treatments of choice. The primary tumor is removed by amputation or limb-salvage procedures. We know that greater than 90% of tumors have spread microscopically to the lungs (i.e. cannot be detected on initial radiographs) at the time of the initial diagnosis. Therefore, if amputation or limb-salvage is performed but not followed with chemotherapy, patients will not gain a survival advantage. The reason for this is that the microscopic disease already present in the lungs grows. The purpose of chemotherapy is to prevent or delay this growth.
How is chemotherapy administered?
Several drugs have been shown to be effective in treating OSA in dogs. We recommend either cisplatin or carboplatin alone or in combination with doxorubicin. The first chemotherapy is ideally administered immediately following surgery, and then further treatments are given at 21 day intervals for a total of 5-6 treatments. Treatments are administered intravenously. In generally, chemotherapy is exceptionally well tolerated in pets. In cats, OSA tends to remain localized, therefore chemotherapy is not advised.
What is the prognosis for OSA?
With OSA, a cure is possible, but in dogs actually occurs less than 20% of the time. Prolonged remissions of 1-2+ years are not uncommon. Median survival for patients treated with surgery and chemotherapy is approximately 13 months, with 35% of patients still alive and disease free at 2 years. Many patients survive much longer periods of time, but it is important for pet owners to understand the statistics for this disease so that they enter treatment protocols completely informed. The quality of life during and after treatment is usually excellent.
Radiation therapy can be used in patients who are poor candidates for limb-salvage or amputation. A recent study showed that >90% of dogs treated with 4 dosages of radiation therapy experienced significant pain relief that lasted a median of 7 months. Chemotherapy combined with palliative radiation can actually improve survival times.
A group of drugs that help decrease bone pain in primary or metastatic bone cancer and may help delay the progression of the disease. These drugs are believed to work through several mechanisms, including inhibiting bone resorption, impeding osteoclast (cells that break down bone) activity, inducing apoptosis (programmed cell death) in osteoclasts (cells of malignancy in OSA), and also may exert an anti-angiogenic effect (inhibiting blood vessel formation that helps tumors grow and spread). A commonly used bisphosphonate is a drug called Pamidronate. It is given as a 2 hour IV infusion once monthly. Pamidronate can be used alone for palliation, or in combination with radiation, chemotherapy, or definitive therapy for OSA.
OSA as described by people with this disease, is very painful. Removing the source of pain through amputation or limb-salvage provides the most complete pain relief. When this is not possible, a variety of analgesics can be used. Non-steroidal anti-inflammatory drugs (NSAIDS) are usually the first line medications used. When NSAIDS alone do not provide adequate analgesia, they are usually combined with other medications such as Tramadol,amantadine, gabapentin, transdermal fentanyl patches, to name a few.