Vaccine-associated sarcoma is a term used for tumors that occur at sites where primarily feline leukemia and rabies vaccines have been administered in cats. Vaccine site sarcomas have not been reported in other species such as dogs and horses. In cats, however, more than 600 cases have been described in the literature and the prevalence of this complication is 1 per every 1,000-10,000 feline leukemia or rabies vaccine administered. Cat owners and veterinarians often ask why the cat is susceptible and other species such as dogs are not susceptible to injection site tumor development following rabies or other vaccination. It has been known for some time that cats are extremely susceptible to oxidative injury when compared to other species like dogs or humans. The cat's increased sensitivity to oxidative stress is recognized clinically by the cat's susceptibility to Heinz body anemia, steatitis (inflammation of fat), and Tylenol toxicity.

We now know that certain vaccines that contain adjuvants, primarily FeLV and rabies vaccines, produce chronic inflammation and free radicals at the injection site. Nonadjuvanted vaccines do not produce either free radicals or chronic injection site inflammation. The free radicals result in oxidative injury to cellular DNA in this susceptible species. The inflammation stimulates cells like fibrocytes to divide. Cats are not only extremely sensitive to oxidative injury but some cats have defects in their tumor suppressor gene called P₅₃. In cats with defective P_53, cells in which DNA has been damaged by free radical formation are allowed to divide. It is this proliferation of defective cells that is believed to spawn vaccine site sarcomas. Industry has responded to the recent knowledge of the mechanism of vaccine-associated sarcomas and new rabies vaccines that do not contain adjuvants have recently been approved by the USDA for use in cats. It is hoped that with the use of safer vaccines, the incidence of this tumor may be reduced significantly.

Since the time from vaccination until tumor development may be as long as 11 years, the vaccine-associated sarcoma problem is not expected to go away anytime soon, and veterinarians and cat owners need to know how to manage lumps that occur at vaccination sites.

The National Vaccine-Associated Sarcoma Task Force has made specific recommendations on how to manage local postvaccinal reactions. Since some vaccines can produce lumps in essentially 100% of cats and only 1/1,000 and 1/10,000 go on to develop a tumor, these recommendations should be used as a guide to postvaccinal lump evaluation and management.

A biopsy and removal of a lump found at a vaccine site should be done if any of the following three situations occur: 1) if the lump remains at a vaccine site longer than three months after vaccine administration, 2) if the lump is greater than 2 cm in diameter at any time after vaccination, or 3) the lump is growing one month after vaccination.

If the lump turns out to be cancerous, early aggressive treatment gives the best results. Surgical treatment requires radical excision of the mass often taking tissue 2-5 cm around the mass. It is recommended that a CT scan or MRI be done prior to surgery to determine the size and depth of tumor invasion in order to most consistently obtain 2-5 cm margins.

Despite aggressive surgery, 60% of these tumors recur with surgery alone. Radiation therapy is now considered an important part of the treatment regimen and can reduce recurrence to 30-40% when combined with surgery. Chemotherapy has been used to further enhance surgery and radiation and decrease recurrence approximately another 10%.

Because treatment is expensive and less than 100% successful, prevention and techniques that enhance success of treatment should be emphasized. One such recommendation is the administration of rabies and feline leukemia vaccines under the skin in the rear legs. Although this practice will not decrease sarcoma development, it will allow earlier detection of cancer and more aggressive surgery and a higher probability of cure.