Bone Cancer in Cats (Osteosarcoma): Causes, Symptoms, & Treatment

What Is Bone Cancer in Cats?

It’s important to understand the terminology when it comes to cancer in dogs and cats, just as it is in humans: such diseases have a universal language that is shared between species.

• “Cancer” is the medical term used to describe an aggressive type of neoplasia (from the Greek for “new cells”) or tumor (from the Latin for “a swelling”).
• Neoplasia (or tumors) describe a disease process where a group of abnormal cells somewhere in the body start to multiply in an uncontrolled way, causing a growth or swelling that is made up of these abnormal cells. These cells can develop almost anywhere in the body, from almost any type of cell.

Neoplasia/tumors are divided into benign types and malignant types. Cancer is the term that is used to describe malignant neoplasia or malignant tumors.

• Benign neoplasia/tumors do not spread to other parts of the body, and do not invade surrounding tissue, so they are relatively easy to treat and cure. Usually, surgical excision is all that’s needed.
• In contrast, malignant neoplasia/tumors, also known as cancer, tend to spread to other parts of the body via the bloodstream and lymph system (so-called “metastasis), they tend to invade adjacent lymph nodes and tissues, and they tend to be difficult to treat and to cure. Surgical excision is often not enough.

When it’s bone cells that develop malignant neoplasia/tumors in this way, this is known as bone cancer. Bone cancer can be primary or secondary.

• Primary bone cancer or primary bone tumors describe the situation where the cancer first develops in the cells of the bone itself. There are several different possible cells of origin: bone cells (osteosarcoma), cartilage cells (chondrosarcoma), fibrous tissue cells (fibrosarcoma), bone marrow cells (haemangiosarcoma), or sometimes other cells (such as squamous cell carcinoma, invading bone tissue). The first of these – osteosarcoma (abbreviated to OSA), constitutes up to 80% of cases of bone cancer. The other soft tissue sarcomas are the next most common group.
• Secondary bone cancer means that the cancer starts elsewhere in the body, and spreads (metastasizes) to the bones. This is less common than primary bone cancer.

Primary bone cancer can metastasize (spread to elsewhere in the body, such as to the lungs) but metastasizing does not happen as often in cats (5 – 10% of cases) as it does in dogs (over 90% of cases).

Around half of the cases of bone cancer affect “appendicular skeleton” which means the long bones of the limbs, while the other half affect the “axial skeleton” which means the skull (especially the mouth, such as the lower jaw or mandible) and the spine. The most commonly affected limb bones are distal femur (lower thigh bone), proximal tibia (upper shin bone), humerus (upper bone of the foreleg) and the digits (toes).

Bone cancer is usually a disease of middle-aged or older cats, with signs typically starting at around 9 years of age. The incidence of primary bone cancer is around 3 – 5 cats in a population of 100,000 cats.

Also Read: Cancer in Cats: Causes, Symptoms, and Treatment

Symptoms of Bone Cancer in Cats

The symptoms of bone cancer in cats include physical swelling, lameness, an abnormal gait, and general unwellness.

Bone cancer causes an abnormal swelling to develop in the affected part of the skeleton. Bone cancer causes a number of signs of unwellness, for two main reasons.

• The cancer causes pain
• The physical swelling of the cancer causes mechanical problems (physically getting in the way of normal function).

Clinical signs noticed by owners include the physical swelling itself, as well as lameness, an abnormal gait, and general unwellness in the affected cat. Affected cats may become dull, inappetant and slow to move around.

Often the lameness is severe, with the cat refusing to place the affected limb on the ground. If the bone cancer is in the mouth, the cat may not wish to eat at all. Weight loss is a common sign.

Diagnosis of Bone Cancer in Cats

If your DVM veterinarian suspects that your cat may have bone cancer, the following steps may be taken, and at some stage in the process, a referral to a veterinary oncologist may be recommended.

Detailed History Taking

Your vet will discuss every aspect of your cat’s life. There are other reasons why cats can develop similar signs to bone cancer, and this history will help to differentiate the various possible causes. Examples include bone infections, injuries, and fights.

As an example of how complex this can be, an owner may think that their cat has broken a bone in a minor accident. However, bone cancer can weaken the structure of the bones, leading to a type of broken bone known as a pathological fracture.

In such cases, it’s only when x-rays are taken that it becomes clear that the bone had been weakened by the cancer before the fracture took place.

Physical Examination

Your veterinarian will check your cat over carefully, ruling out other causes of the signs that are being shown.

Routine Blood Tests and Other Laboratory Work

Your veterinarian may suggest blood tests, including the usual panel of diagnostic tests, such as hematology (blood count) and biochemistry profiles, to confirm that there is no other underlying illness affecting your cat. Urinalysis may also be carried out.

Biochemistry changes in the blood, including elevation of the alkaline phosphatase (AlkP) enzyme, may be seen if a cat is suffering from bone cancer.

Diagnostic Imaging

• Radiographs: X-ray pictures of the affected area are the key to making a tentative diagnosis of bone cancer. A distinctive pattern of radiographic changes is seen, including so-called “lysis” (blackened areas indicating destruction of areas of bone) and increased whitened areas indicating abnormal new bone formation caused by cancerous tissue.
  Sometimes, these changes can be difficult to tell apart from other disease processes (such as bacterial or fungal infections of the bone), and other tests may be suggested to confirm the diagnosis.
• Nuclear Scintigraphy: bone scans can be a sensitive tool for detecting areas of active new bone formation, which can point towards areas of bone cancer that may not have been previously noticed.
• Positron Emission Tomography -Computed Tomography (PET-CT): This method of imaging highlights areas of increased glucose uptake in the body, which can be a useful way of identifying both primary and secondary sites of bone cancer. This can be an effective way of identifying metastatic disease in particular.

Bone Biopsy

Your veterinarian may suggest different ways of using a biopsy of the lesion to confirm the tentative diagnosis of bone cancer.

• A fine needle aspirate (FNA). This simple method collects a small number of cells from the affected area that can be sent off to the laboratory for analysis known as cytology. This is not usually enough to make a definitive diagnosis, but it can give supportive evidence.
• A full biopsy, which is a more complex, invasive process, allows histopathology to be carried out, which is the best way of definitively confirming the diagnosis of bone cancer, as well as allowing the precise type of bone cancer to be identified. While a biopsy can be collected as a stand alone surgical procedure on an affected cat, sometimes it may be carried out on a sample from a limb that has been amputated because of strong suspicions of bone cancer.

Treatment Options for Bone Cancer in Cats

Treatment of bone cancer in cats comes in several forms, but surgical removal of the affected area is the first line of treatment.

• Surgical removal of the affected area, with a margin of surrounding healthy tissue, is the first line of treatment. This nearly always means amputation of affected limbs which can be an emotionally charged decision for some cat carers. In rare cases, so-called limb-sparing surgery may be suggested as a way of preserving the limb while still removing the totality of the cancer. An orthopedic specialist surgeon may be needed to carry out this type of complex procedure.
• Chemotherapy is not usually recommended, as side effects are likely, and it does not extend survival times significantly.
• Radiation therapy, or stereotactic radiation therapy, as a follow up to surgical removal of the cancer, may sometimes be recommended in specific cases.
• Pain relief and anti-inflammatory medication is important in cases of bone cancer, and in particular if surgical excision is not going to be carried out, this is central to palliative treatment.

Monitoring and Prognosis

Complete surgical removal of the cancer (meaning amputation of the limb in cases affecting this part of the body) results in survival times of 24-44 months from the time of diagnosis.

No additional therapies have been proven to be effective in extending these survival times.

It is more difficult to surgically remove bone cancer when it is located in the skull and spine: the average survival time for these patients is shorter, at around six months.

Conclusion

Bone cancer is a serious, life-limiting disease of middle aged and elderly cats. Early diagnosis and, when possible, aggressive surgical removal of affected areas is the best way of extending the lives of affected cats.