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One of the greatest worries of all pet owners is the thought of cancer affecting their pet. Pet owners a few years ago became aware of, and justifiably concerned about, a condition called feline vaccine-associated sarcoma (VAS).

In the last 15 years, the veterinary community has come to the realization that some of the soft tissue tumors they are encountering in cats may be associated with previous vaccine administration. These soft tissue tumors are difficult to treat, have a high incidence of recurrence, and can lead to death in some cats. A cat positive for the feline leukemia virus is particularly at risk for developing multiple tumor types including fibrosarcoma.

What Causes Some Cats to Develop Fibrosarcomas at Injection Sites?

Tumor stimulation is thought to be secondary to a chronic inflammatory reaction that a predisposed cat cannot control rather than a specific reaction to the vaccine virus component itself. Not all soft tissue tumors are sarcomas and not all of these tumor types are associated with injection sites. In fact, the majority of them are not clearly associated with any injections.

The genetics of the cat appears to affect the propensity for the reaction to progress from inflammation to tumor, and a comprehensive survey study published in November 2003 was unable to establish that any vaccine product was specifically a risk factor for tumor formation, though earlier studies had pointed toward leukemia and rabies vaccines as being most frequently associated with this adverse reaction.

Frequency rates of sarcomas are reported to be about 0.32 to 2 cases per 10,000 doses given, so they are very rare. Sarcomas can spread from the site of the injection down into surrounding connective tissue, muscle, and bone. Even with chemotherapy, radiation, and surgical removal some cats die from vaccine-associated sarcomas.

As the recognition of this disease has grown, clients have begun to question the vaccine protocols that have been historically recommended by their family veterinarian. This issue has been of great concern to both practitioners and cat owners and has led to the creation of a professional study group called the Vaccine-Associated Feline Sarcoma Task Force, which studied vaccines and vaccine recommendations and collated a wealth of information from experts in the area of feline oncology regarding tumor detection and treatment.

Changes Recommended by Task Force

Vaccines and medications used to be administered in the inter-scapular area (between the shoulder blades) at the back of the neck most often, and this was also where the tumors were usually found. Many types of material, even embedded surgical stitches (sutures) can cause the irritation cycle necessary to encourage tumor development, but vaccines are the best studied. Recently, changes in vaccine formulation to change or eliminate the adjuvant (the part of vaccine that “calls” cells to react to the injection for a time after it is administered) or simplify vaccine complexity by using genetically engineered subunits have been carried out. Some new vaccines can even be given without needle, instead using an air injection to penetrate the skin. Research continues into production of less reactive vaccines, and veterinarians have now adopted new vaccination protocols to help reduce the number of vaccines administered over the lifetime of the cat.

According to Colorado State veterinary oncologist Gregory Oglilvie "vaccine/injection associated malignancies have been reported to develop within weeks to years after injection…the more often a cat is vaccinated, the higher the risk of vaccine-associated sarcomas."

Diagnosis and Treatment of Vaccine-Induced Sarcomas

If a cat owner notes a lump in the area where vaccines have been administered, it is imperative to seek veterinary care. Most are just benign irritation swellings (granulomas) that disappear in a week or two at most, but if they swell further or do not disappear, monitoring or diagnostics will begin. Tumors can appear as early as 3 months post-injection or take 2-3 years to develop.

Because of the aggressive nature of these tumors, the diagnosis and treatment is complex. Whether the cat has feline leukemia virus will also affect the outcome—these cats frequently have multiple tumors that behave very aggressively. These may be quite young cats, also.

Once a veterinarian has assessed the non-regressing lump and performed a full physical examination, the next step is to confirm the nature of the lump as cancer versus a pus pocket, foreign body, irritation, fatty deposit, or other benign event. A fine needle aspirate and/or surgical biopsy is generally carried out next, though if suspicion is strong that this is an early tumor, immediate radical excision is done. The biopsy sample will be sent to a clinical pathologist/oncologist for confirmation of diagnosis.

Once the diagnosis of sarcoma is made (or any other malignant tumor), the next step is to determine if there is metastatic disease. Metastatic refers to the spread of the tumor from a single location to somewhere else distant in the body. X-rays of the chest and the area of the tumor will help check for these metastases. Local lymph nodes and the lungs are two common sites for metastasis. Any lymph nodes that are enlarged will be biopsied as well. Ideally, magnetic resonance imaging (MRI), and if extension into bone is a concern, computed tomography (CT) is used to determine the full extent of the primary tumor. However, this type of imaging tool is not available to the average feline practitioner but requires referral to an equipped specialty practice or university veterinary teaching facility.

The general health of the patient must be assessed as well. Some combination of a complete blood count, general chemistry profile, urinalysis, thyroid assessment, feline leukemia virus and feline immunodeficiency virus tests will be recommended by the veterinarian. These tests allow the practitioner to get an accurate assessment of the patient’s overall health and will help determine the treatment and prognosis for a cat afflicted with sarcoma.

During surgical removal, extra tissue surrounding the tumor is also removed to help ensure that all cancerous cells are excised. According to Dr. Ogilvie, "computerized tomography and magnetic resonance imaging are often valuable tools to help direct the tumor excision."

Radiation therapy is generally available only at referral specialty facilities and, therefore, may not be easily accessible. There has been some research on the use of immune stimulants to control the growth of these tumors, but because of the lack of scientific data proving their worth, surgical excision and radiation remains the initial treatment of choice for small to medium sized tumors. With combined surgery and radiation and chemotherapy as opposed to surgery alone, the chances for survival increase.

Large tumors are of concern. These tumors may encompass big amounts of soft tissue or bone and may need to be reduced in size with chemotherapy or radiation (debulked) before they can be surgically removed, or they may be completely inoperable. These tumors carry a much worse prognosis. Recurrence rates for surgery even with radiation and chemotherapy is 30-70%, and spread of the tumor (metastasis) occurs in 10-25% of affected cats.

Aftercare of the Sarcoma Patient

The initial period after surgery and/or radiation and chemotherapy treatment can be a very tough time for the patient. Pain control medication is used to keep the cat comfortable. The incision must be kept clean and dry, and the cat must be monitored closely for any signs of fever, swelling, or discharge. Common adverse effects from radiation can be hair loss or change in hair or skin color. The most common side effects from chemotherapy are loss of appetite, low cell count, and nausea. Medications can be given to control vomiting, and the patient must be monitored carefully for dehydration and weight loss.

In consultation with your veterinarian, each client must carefully consider the prognosis for long term survival when deciding whether to proceed with a cat’s treatment for sarcoma. The average time elapsing before recurrence of the tumor can be just weeks to months. There are some cats that have survived greater than two years after treatment, but most had small early tumors combined with radical surgical excisions and were treated with radiation and chemotherapy. The decision to treat will be made after considering each patient’s specific condition. Cost, prognosis, and patient welfare will be considered.

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