How To Treat Osteosarcoma When Surgery Is Refused
A dog with distal tibial osteosarcoma undergoes palliative radiation therapy using a linear accelerator.
Photo Courtesy of Dr. Michael Keat, UC Davis
Osteosarcoma (OSA) is a common and devastating cancer that plagues many of our canine patients. Standard-of-care therapy continues to include the recommendation for surgical removal of the primary tumor through amputation or through limb-sparing surgery followed by chemotherapy.
However, some patients may not be good candidates for amputation or their owners may prefer to pursue non-surgical alternatives. Recent years have brought new radiotherapy techniques as well as new options for pain control.
External beam radiation therapy has been studied as a palliative treatment for OSA for many years and is used strictly to impart local pain control. Total radiation doses of 24 to 36 Gy are administered to the tumor site over three to four weekly fractions. Significant pain relief is expected in 74 to 92 percent of dogs for a median of two to three months, although most dogs will still require some pain medications.
Skin reactions are mild with this palliative dosing scheme and generally do not affect quality of life. Local tumor control is not expected with this treatment modality and, occasionally, pathologic fracture may occur at the tumor site. This treatment option is available at many universities and specialty practices across the country.
A Promising Modality
Another type of radiation therapy termed stereotactic radiosurgery, or SRS, is a new non-surgical treatment option available to dogs with OSA.
First reported in dogs in 2004 by the University of Florida, this modality administers a very high dose of radiation to the tumor while sparing most of the normal surrounding tissues. SRS entails either a single high dose or a few slightly smaller doses with the help of highly advanced treatment planning equipment.
SRS can result in necrosis of a high percentage of tumor cells, a result not possible with traditional palliative radiation therapy. While further study is necessary, SRS may be able to provide long-term pain control as well as tumor control without the need for surgery.
Pathologic fracture is still a problem with this type of therapy and may be delayed by four to six months after treatment.
This treatment modality is available at the University of Florida, Colorado State University and the Animal Specialty Center in Yonkers, N.Y.
Samarium 153 lexidronam is a radioisotope bound to a salt that can be administered intravenously and concentrates in areas of increased osteoblastic activity. Treatment resulted in pain reduction in 63 percent of dogs with bone tumors in one study, but did not prevent local tumor progression or extend survival time. It seems to be most effective in dogs with small tumors.
Three to four days of hospitalization is required after treatment while radioactivity within the patient resolves. Neutropenia or thrombocytopenia occurred in the majority of patients treated. Treatment with Samarium 153 lexidronam can be repeated safely if necessary. This therapy is available at the University of Missouri, Colorado State University and the Veterinary Specialty Center in Buffalo Grove, Ill.
A lot of interest has been shown recently in the use of bisphosphonates in dogs with OSA. This class of drugs, which includes pamidronate and zoledronate, act as osteoclast inhibitors and are used extensively in people with metastatic breast or prostate cancer to bone.
Bisphosphonates appear to also have some direct anti-neoplastic effects. In a recent study, 30 percent of dogs with OSA treated with pamidronate achieved durable pain control for a median of 7.7 months. Pamidronate is administered as a two-hour intravenous infusion with saline every four weeks and has an extremely low risk of side effects. Bisphosphonates may be used in combination with radiation therapy to increase the chances of achieving adequate pain control.
The importance of vigilant medical pain control as a viable short-term treatment option should not be underestimated. Non-steroidal anti-inflammatory drugs (NSAIDs) continue to be the cornerstone of treatment for patients with OSA who do not undergo surgery, and more drug choices are available now than ever.
NSAID therapy alone, however, is rarely adequate to control pain and improve mobility in dogs with OSA. The addition of Tramadol is usually necessary even in early cases and, because it has a wide dose range, it can be titrated to reach the ideal effect in each patient.
At high doses, this drug may cause sedation, agitation or gastrointestinal side effects. Amantadine or gabapentin may be added to the combination of an NSAID and Tramadol in patients with acute pain, and both are generally well tolerated.
Should non-controlled drugs fail to achieve the desired degree of pain control, codeine, extended-release morphine or other oral opiates may be prescribed. Sedation and constipation are common side effects of these drugs, however, and the patient’s quality of life should be seriously considered before long-term use of these drugs is recommended.
At the University of California, Davis, veterinary medical teaching hospital, we always encourage routine recheck examinations even for patients with OSA receiving only pain-control medications. This allows us to reassess pain levels and modify the treatment plan as the tumor progresses locally.
In summary, many options other than surgery exist for veterinarians who treat dogs with OSA. Most of these treatments focus on pain control and quality of life, important considerations for any cancer patient.
Research in this field is ongoing. Additional radiotherapy and medical options as well as other novel therapies are likely to become available within the next several years.
Dr. Skorupski, Dipl. ACVIM (oncology), is assistant professor of clinical medical oncology at the University of California, Davis.