Out on a limb: osteosarcoma in dogs and cats

Although owners may balk at amputation, the treatment removes the patient's source of pain and most dogs fare well on three legs. (Saharrr/stock.adobe.com)

Osteosarcoma remains the most common primary bone tumor in dogs and cats. At Fetch dvm360 conference in Baltimore, Kim Johnson, DVM, DACVIM, explained that this aggressive form of cancer requires adequate pain management and a focus on patient quality of life.

Pathogenesis of canine osteosarcoma

Osteosarcoma accounts for an overwhelming majority (85%) of canine bone tumors, said Dr. Johnson, who is CEO and founder of Pet Cancer Consulting in Nashville, Tennessee. Although the pathogenic mechanisms of this neoplasm aren't fully understood, osteosarcoma is influenced by abnormal bone cell growth and hormonal stimulation. Tumors also frequently develop at the location of a previous fracture, likely because the rapid rate of cell proliferation associated with healing increases the risk of genetic mutation. Old reports of metal hardware influencing osteosarcoma development have decreased because contemporary surgery methods use different metals than those implicated in tumor development.

Although certain breeds are predisposed to developing primary bone tumors, Dr. Johnson reminded the audience that “all dogs have bones and can get osteosarcoma.” Dogs at high risk include large and giant breeds, especially Rottweilers, retrievers and doodle crosses. Canine osteosarcoma is usually a disease of middle and old age, she said, although young dogs tend to develop highly aggressive tumors. Appendicular osteosarcoma is the most common form, with tumors developing “away from the elbow and toward the knee.” The forelimb and hind limb are the most common tumor sites for large and small breeds, respectively. Although relatively uncommon, osteosarcoma can also affect the skull, ribs, vertebrae and pelvis.

Locally destructive tumors with aggressive spread

Early signs of osteosarcoma are typically nonspecific and may include pain, lameness, toe touching and local swelling. Dogs may experience loss of appetite, weight loss and exercise intolerance. Signs may also reflect the tumor's location; for example, a nasal tumor may cause epistaxis and a vertebral tumor may induce neurologic signs.

Due to this neoplasm's highly aggressive and locally destructive biology, pain typically worsens over time and can cause dogs to become irritable or aggressive. In Dr. Johnson's experience, dogs usually present for diagnostic workup after an inciting event, such as sudden lameness or a pathologic fracture. Ninety percent of canine osteosarcoma tumors metastasize, and dogs with pulmonary metastases may experience respiratory signs.

Diagnosis affects prognosis (but rarely treatment)

Osteosarcoma-associated pain should not be underestimated, Dr. Johnson warned. She recommended initiating nonsteroidal anti-inflammatory drug (NSAID) therapy right away to help relieve discomfort during the diagnostic period.

Differentiating osteosarcoma from other tumors (e.g. fibrosarcoma, chondrosarcoma) and fungal infection will influence the prognosis more so than the treatment course, as many diagnostic differentials still necessitate amputation.

The initial diagnostic workup should include a complete blood count and serum biochemistry profile, as a persistently elevated alkaline phosphatase (ALP) level even after amputation worsens prognosis. On radiographs, osteosarcoma should not cross the joint, and the tumor will typically have a proliferative exterior and lytic interior. (As an exception, osteosarcoma in greyhounds tends to appear lytic throughout.) Codman triangle, a radiographic sign of periosteal reaction, is now considered an unreliable indicator of osteosarcoma.

Three-view thoracic radiographs and an abdominal ultrasound should also be performed to look for evidence of metastasis. Dr. Johnson reminded the audience that, while only 10% of canine osteosarcoma patients have visible metastatic tumors at the time of diagnosis, 80% to 90% already have micrometastasis.

Other possible diagnostic tools include nuclear scintigraphy to visualize areas of injury and inflammation, although she cautioned that tumors may appear either normal or as reactive inflammation on a bone scan. Metastatic disease usually causes pinpoint areas of radionuclide uptake, whereas symmetric areas of uptake suggest arthritis.

If you choose to biopsy a suspected osteosarcoma tumor, Dr. Johnson advises to “go big or go home. Get as many pieces as you can for the pathologist.” She always discusses the risk of pathologic fracture with clients but says the risk shouldn't keep practitioners from performing biopsies. “If a limb fractures with biopsy,” she explained, it “was likely going to fracture within a few days anyway.”

Treatment options: palliative and aggressive

Without treatment, survival after osteosarcoma diagnosis may be as low as 1 to 2 months, especially if metastasis is evident. Additional negative prognostic indicators include young age, large tumor size, high histopathologic grade, humeral location and/or elevated serum ALP level.

“I support my clients who do nothing, who do surgery or who do medical management.”

Osteosarcoma treatment aims to address both “life and limb” by managing local pain and slowing disease progression. Dr. Johnson emphasized that both aggressive and palliative options are acceptable. “I support my clients who do nothing, who do surgery or who do medical management,” she said. No matter which treatment course they pursue, “We're still a team.”

Gold standard treatment consists of amputation and chemotherapy and presents a median survival time (MST) of 12 to 18 months. Carboplatin or doxorubicin chemotherapy is typically administered every 3 weeks for four to six treatments. Alone, medical management or amputation each offers a 4- to 6-month MST, which reduces to 1 to 3 months if significant lung metastasis is present.

Clients may be reluctant to amputate, especially in cases of early diagnosis. In these situations, Dr. Johnson suggests taking radiographs, managing pain and repeating imaging in about a month to track tumor growth. Single or combination oral analgesic therapy may include an NSAID (e.g. Rimadyl), tramadol, gabapentin or amantadine.

Many clients balk at the prospect of limb removal because the procedure reportedly doesn't extend survival. However, Dr. Johnson noted that amputation dramatically restores patient quality of life by “effectively removing the source of pain.” After the initial recovery period, amputation is typically well tolerated by patients, and clients often wish they had agreed to the procedure sooner.

Dr. Johnson noted that an orthopedic examination should be performed before amputation to ensure the patient's other limbs can provide adequate support. If a pet's large size or concurrent diseases make amputation impossible, then limb-sparing therapies, such as radiation or bisphosphonates (e.g. pamidronate), may be attempted. Each method reportedly extends MST to 6 to 9 months; however, adjunctive chemotherapy is still recommended to slow disease progression.

Feline osteosarcoma: cats are not small dogs

Cats have a lower risk than dogs of developing primary bone tumors, Dr. Johnson said. “Osteosarcoma accounts for 70% of feline primary bone tumors, and cats of all ages can develop this neoplasm,” she said. Tumors are more common in the hind limb than the forelimb, and frequent sites include the distal femur, proximal tibia and proximal humerus.

Clinical signs are similar to those observed in dogs and typically include local swelling, pain, and reduced range of motion. Feline osteosarcoma tends to be locally aggressive but, unlike canine osteosarcoma, has low metastatic potential. Therefore, amputation is the treatment of choice, and adjunctive therapies (including chemotherapy) are rarely indicated. A 5-year survival period after surgery is common.

Dr. Stilwell provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting. In addition to her DVM obtained from Auburn University, she holds a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida.