Osteosarcoma in Cats

By Krista Williams, BSc, DVM, CCRP; Debbie Stoewen DVM, MSW, RSW, PhD; Christopher Pinard, DVM

What is osteosarcoma?

Osteosarcoma is a malignant tumor of the bone. This cancer has the same appearance as human pediatric osteosarcoma. Osteosarcomas are tumors that arise from the abnormal production of cells that create and break down bone (called osteoblasts and osteoclasts, respectively).

The long bones of the legs are the most commonly affected, though bones such as the skull, pelvis, ribs, or vertebrae may also be affected. Osteosarcoma can also affect non-bony tissues, including the mammary glands, spleen, liver, and kidneys. This is called extraskeletal osteosarcoma.

What causes this type of tumor?

The reason why a particular pet may develop this, or any tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary.

What are the signs of osteosarcoma?

Osteosarcoma is very painful. If your cat has an osteosarcoma of the limb (appendicular osteosarcoma), lameness or a distinct swelling may be noted. Your cat may be more lethargic, have loss of appetite, and be reluctant to walk or play due to pain caused by the tumor on the bone. The most common location where osteosarcomas develop in cats is the hindlimb (femur and tibia).

"Your cat may be more lethargic, have loss of appetite, and be reluctant to walk or play..."

Osteosarcomas can also develop in the skull (most commonly in the oral cavity), pelvis, ribs, and vertebrae. Cats with osteosarcoma of the jaw (a form of axial osteosarcoma) may have swelling of the jaw, difficulty opening the mouth, excessive salivation, and may be reluctant to eat due to pain. If an osteosarcoma develops elsewhere in the skull or vertebrae, swelling and pain in the head or along the back may be observed.

How is osteosarcoma diagnosed?

Most cats with osteosarcoma have lameness of a limb. Swelling is usually noted where the tumor has grown, and the area will be warm to the touch due to a tremendous amount of inflammation. Your veterinarian will take X-rays of the region. Osteosarcomas appear lytic (meaning pieces of bone are missing) or ‘moth-eaten’ due to the loss of normal bone tissue. Fractures can be present if the bone has weakened enough.

Once a lesion is suspected, a more definitive diagnosis may be obtained by a fine needle aspiration. This involves taking a small needle with a syringe and suctioning a sample of cells directly from the lesion and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope. This is performed under sedation. If this procedure is not diagnostic, a bone biopsy may be warranted. In most cases, lytic bone lesions found on X-rays are indicative of either an infectious or malignant process and further diagnostics are always recommended.

How does this cancer typically progress?


Osteosarcoma in cats is less aggressive, and metastasis (spread to other areas of the body) is rare; however, staging (searching for potential spread to other locations in the body) is recommended. This may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If any lymph nodes are enlarged or feel abnormal, further sampling may be pursued to determine if spread is present.

What are the treatments for this type of tumor?

Without evidence of spread, the primary goal is local tumor control. This typically involves amputation of the affected limb. Though this is unsettling for many cat owners, most cats do very well after amputation. Surgery is almost always pursued as long as it is a safe and viable option.

Given that control of the disease is usually achieved by amputation, chemotherapy is not typically pursued.

Is there anything else I should know?

Adequate pain control is of utmost importance. Discuss appropriate pre- and post-operative therapy and pain management plans with your veterinarian.

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