Osteosarcoma in Cats

Quick Facts at a Glance

• Average age of onset is 10.2 years (range : 4-14 years)
• Osteosarcoma occurs in appendicular, axial and extraskeletal sites
• Appendicular and axial osteosarcoma are primarily of medullary origin
• Appendicular osteosarcoma affects hind limbs more often than front limbs
• Skull and pelvis are the most common axial sites
• Osteosarcoma arising at a prior fracture site has been reported in 6 cats
• Extraskeletal osteosarcoma has been reported in mammary and ocular sites
  

Signalment and presenting signs?

Osteosarcoma in cats is reported to occur in appendicular, axial and extraskeletal sites with similar incidence. Cats with axial osteosarcoma tend to be a little older at the time of diagnosis than cats with appendicular osteosarcoma (mean of 8 years vs. 10.5 years respectively). Males and females appear to develop osteosarcoma with equal frequency. A breed predilection for osteosarcoma does not exist. Appendicular osteosarcoma occurs more often in the long bones of the hind limbs than in the front limbs. Primary involvement of the digits has been reported. Axial osteosarcoma most commonly affects the skull (maxilla, mandible) and the bones of the pelvis. Extraskeletal osteosarcoma is found most commonly in subcutaneous, mammary and ocular tissues. Extraskeletal osteosarcoma (ESOS) has been reported to develop at sits of prior vaccination. Prior vaccination at the tumor site has been determined to be a statistically significant risk factor for the development of ESOS. Symptoms at the time of presentation are directly related to the location of the primary osteosarcoma.

What should the diagnostic evaluation include?

As with any feline cancer patient, the initial diagnostic evaluation consists of a CBC, biochemical profile, UA, T4 and viral status. Thoracic radiographs for evaluation of pulmonary metastasis is indicated at the time of identification of the mass. An incisional biopsy allows for confirmation of a histologic diagnosis. The biopsy is a critical step because it provides the information necessary to adequately plan treatment. Submission of the entire tumor with surrounding normal tissue for complete histopathologic evaluation is essential to determine the success of resection. If tumor cells are confirmed to extend to any of the surgical margins, additional therapy is recommended.

What is the recommended surgical approach?

Amputation is the treatment of choice for appendicular osteosarcoma in cats. Because of the medullary origin of osteosarcoma, extensive marrow involvement can be anticipated. Amputation should therefore be performed at the level of one joint above the tumor. When osteosarcoma affects the distal femur or below, disarticulation of the coxofemoral joint is recommended. When the proximal femur is affected, partial pelvectomy is required to achieve clean surgical margins. When the tumor affects the humerus, amputation with scapulectomy is recommended.

Radical surgical resection of axial osteosarcoma is recommended. Location of the tumor limits the ability to achieve complete tumor resection while still maintaining function and quality of life. Mandibular osteosarcoma is appropriately treated with hemimandibulectomy because of the medullary origin of these tumors. Tumors of the maxilla, vertebrae and pelvis are often diagnosed at a stage of disease that is too advanced to allow for adequate resection. Radiation therapy can be beneficial to control local disease in these cases.

The ability to completely resect extraskeletal osteosarcoma is also dependent on tumor location. Interscapular ESOS presents the same surgical challenges as other types of vaccine associated sarcomas. Radiation therapy after surgery is recommended. ESOS of other subcutaneous sites and of mammary tissue are often amenable to wide surgical resection

What is the survival time following surgery?

The median survival time for cats with appendicular osteosarcoma undergoing amputation has been recently reported as 16.7 months (range: 1-60 months). This is in contrast to the results of a small but widely cited study, published more than a decade ago, reporting median survival times of 49 months after amputation. Fifteen percent of cats live greater than 2 years following amputation alone.

Survival times for cats with appendicular osteosarcoma are very short when amputation is not done. The average survival time for cats with ESOS undergoing surgery s 12.67 months. The location of the ESOS and ability to completely resect the tumor directly correlates with tumor recurrence and patient survival time. Cats with axial osteosarcoma have the shortest average survival time of only 6 months (range: 1-60 months). Patients with axial osteosarcoma amenable to complete surgical resection have an average survival time of 16 months.

Is adjuvant therapy beneficial?

When complete surgical resection of the tumor is achieved adjuvant radiation therapy is not indicated. Radiation therapy has particular benefit for the treatment of ESOS when complete surgical resection is not possible. Radiation therapy is also beneficial for palliative treatment of appendicular or axial osteosarcoma when surgery is not done or has not been successful.
The benefit of chemotherapy for the treatment of osteosarcoma in cats is unknown. Chemotherapy may be indicated alone or in combination with radiation therapy prior to surgery to maximize cytoreduction and thus enhance the surgical outcome.