Bone Cancer in Cats

Osteosarcoma is the condition of malignant tumors forming in the limbs or within the axial skeleton and causing decay with a potential for metastasizing. Osteosarcoma is considered to be a rare and spontaneous condition overall in cats, and for those who do develop it the tumors are most often benign, yet is still a very painful condition. Its cause is unknown. It is the most common malignant cancer diagnosed in cats and dogs today.

Like humans, cancer in cats manifests as tumors. There are three main forms of bone cancer, but the most common and aggressive is osteosarcoma, which accounts for up to 95% of all cancer diagnoses. Despite this, osteosarcoma is still considered to be a rare cancer in cats. Although cancer can occur anywhere in the body, in cats it usually involves the limbs or any of the bones connecting to the spine such as the ribs, pelvis, and skull. Tumors are classified as either primary, in which the cancer forms directly in the bone, or secondary, where it has spread from an adjacent site. Primary tumors are uncommon in cats, but for those cats who have the tumors up to a third of them are benign. The development of tumors occurs spontaneously with no known or apparent cause.

Common symptoms of bone cancer include:

• Intermittent lameness in any or all of the limbs that becomes more constant over 1-3 months; acute lameness may be due to a bone fracture
• Hard, obvious swelling over a long bone of a limb
• Loss of appetite and significant weight loss
• Progressively worsening pain
• Generalized weakness
• Respiratory difficulties

Symptoms in sites other than the limbs depend on location. For example, tumors near the jaw may cause difficulty in opening the mouth, excessive nasal discharge if it is in the nasal cavity, neurological effects if it is located in the spine, and urinary difficulties if it is on the urinary bladder or prostate.

Types

The primary bone tumors are osteosarcoma, chondrosarcoma, fibrosarcoma, synovial cell carcinoma. Each type behaves differently depending on the grade of tumor.

• Chondrosarcoma: Less aggressive than osteosarcoma and has a lower tendency to spread.
• Fibrosarcoma: Usually localized and has a low metastasizing rate unless the tumor is of high grade.
• Synovial Cell Carcinoma: Grades 1 and 2 sarcomas are potentially curable. Grade 3 usually results in death within several months. 
• Osteosarcoma: This is the most common type in cats and the most aggressive. Potential for cure is favorable through amputation of the affected limb since the metastatic potential in cats is low.

Most primary bone tumors develop unexpectedly with no known or apparent cause, especially osteosarcoma.

There are many key diagnostic tests to positively detect if cancer is present. 

• Physical and orthopedic exam
• Radiographs (x-rays) of the observed area as well as the pelvis and chest to detect metastasis
• Histopathology (tissue changes)
• Chemistry profile to check organ function
• Blood tests, including a CBC
• CT, PET, and/or MRI scans
• Possibly a whole body bone scan with a radioactive marker placed in the bloodstream
• Ultrasound, especially of the heart
• Urinalysis to determine kidney function
• Tissue biopsy

A physical exam will help to assess general health and anti-inflammatory medicine may be prescribed if your cat has just begun to show signs of lameness. If an anti-inflammatory is unsuccessful, an orthopedic exam will be necessary in order to identify which bone is being affected, if there are any other potential causes of lameness such as ligament rupture or neurologic disease, and to determine if the cat will be able to successfully adapt to life on three legs if amputation is being considered. 

X-rays of the chest will help to determine the possible presence of cancer and if the condition has spread to the lungs or if there are any other underlying conditions such as arthritis. The presence of bone cancer shows as a distinct pattern on the x-ray that is highly recognizable. Since initial x-rays typically do not show small metastatic lesions, all cats suspected to have osteosarcoma are treated as if they have metastatic lesions on the lungs. Up to 90% of osteosarcoma tumors spread to the lungs. 

Blood tests, urinalysis, and a chemistry profile will also be taken to check for changes from previous visits. Advanced imaging and scans, along with x-rays, will determine if the disease has spread. 

The only accurate method of making a confirmed diagnosis is a tissue biopsy where the cat is anesthetized and small samples of the bone are extracted for laboratory testing by a pathologist. A whole body scan is recommended to determine metastasis if a nuclear medicine facility is available. Laboratory testing will confirm the type of cancer, the grade of tumor, and if the condition has spread to a regional lymph node.

Traditional treatment of bone cancer is amputation of the affected limb followed by systemic chemotherapy to address any metastasis. Immediate chemotherapy treatment is extremely important since 90% of osteosarcomas spread to the lungs. Radiation therapy has also been found to be effective in controlling pain in some cases.

A limb-sparing procedure may be a possible alternative to amputation if the cancer is found to be in a lower portion of the bone. Here the cancerous area of the bone is removed and replaced with a bone graft from another part of the body. Additional x-rays, advanced imaging and bone scans will be necessary. This procedure has been very successful at reestablishing limb function once the bone graft heals, which is usually in 2-3 months.

You may choose to forgo amputation and chemotherapy. In such cases, large doses of radiation will be administered to alleviate pain. If this option is not chosen, then the only other alternative is euthanasia.

Treatment of osteosarcoma is very personal decision that should be based on your cat’s health and condition and according to what is comfortable for you, the owner.

The type of bone cancer and whether it metastasized will determine your cat’s long-term prognosis:

• Osteosarcoma: With amputation alone, a survival time of 3-5 months is common. Amputation along with chemotherapy has a survival time of about 1-4 years. 
• Chondrosarcoma and Fibrosarcoma: Amputation may be curative without chemotherapy.
• Synovial Cell Sarcoma: Grade 1 and 2 tumors are often cured with amputation alone. Patients with grade 3 tumors usually die within seven months.

Immediately after surgery, your cat will be put in intensive care and intravenous fluids will administered to prevent dehydration. Medication will be given to help control the pain. The length of stay will be about 1-2 days. Your cat should be walking again soon after.

Bandages used during surgery should be kept clean and changed if needed for 2-3 weeks. The surgical site should be checked twice a day for possible signs of infection or breakdown.

Your cat should be encouraged to walk to increase the rate of recovery.

Medications will be prescribed for you to administer at home to control real and phantom pain. Medication may be given for up to one month after surgery. If your cat has a catheter, the medication will need to be administered by you through a port for about two days following surgery.

If you choose chemotherapy, there are potential side effects. The side effects will be different for each drug and your veterinarian should amply and extensively discuss this with you.

Proper nutrition is highly important while undergoing treatment in order to maintain strength, quality of life, increase survival times, and improve response to therapy. It can also help to reduce the length of hospital stay, reduce post-operative complications, and enhance the healing process.

A recurrence of cancer is not common at the initial site where it was found, although possible. Instead, it is more common to reappear at another site if it does at all.

A guarded to poor prognosis is necessary until all the results from diagnostic testing has been completed and analyzed, and until initial response to treatment has been noted. The prognosis largely depends on the severity of the cancer and the extent of metastasis. But because metastasis is not common in cats (less than 10%), the survival rate is very favorable.