William R. Pritchard Veterinary Medical Teaching Hospital

Feline Vaccine Associated Fibrosarcoma

Vaccine associated fibrosarcomas are tumors that arise at sites where cats have been vaccinated. They are most commonly associated with the rabies vaccine and the vaccine for feline leukemia virus. We continue to give these vaccines because of the relatively high risk of contracting these diseases and the relatively low risk of developing a fibrosarcoma. The incidence of these tumors is not known, but is reported to be about 1 in 10,000. Fibrosarcomas tend to be very aggressive locally, and can also metastasize (spread to other areas of the body).

The first attempt at treatment is thought to have the best chance of curing or slowing the disease, although it is possible to treat disease recurrence. Treatment generally involves combinations of surgery, radiation therapy and chemotherapy. It is necessary to determine the extent of the cancer in order to determine which treatment option is best for your cat.

A thorough physical exam, chest radiographs ("x-rays") and blood work are all part of the initial work-up. Although pulmonary metastasis, or the spread of the tumor to the lungs, occurs only about 10-25% of the time, it changes the prognosis and treatment options available for your cat. A CT scan, which allows careful evaluation of the tumor and the tissues around it, is usually required as part of the diagnostic plan. If a biopsy has not been done one will be done at the same time as the CT scan.

With the information obtained from the CT scan we will be able to formulate a treatment plan. If the tumor is very small or non-invasive, surgery alone may be recommended. In other cases we may recommend radiation therapy followed by surgery, or we may recommend chemotherapy alone, which tends to be very well tolerated by cats. If surgery or radiation therapy followed by surgery is recommended we will offer follow-up chemotherapy to try to decrease the likelihood that lung metastasis will occur.

If your cat has a recurrence of a fibrosarcoma, the diagnostic procedures will be very similar. Again, imaging the tumor with radiographs and a CT scan will help us determine the extent of the tumor and help us to formulate a treatment plan for your cat.

This handout is designed to answer some of the basic questions relating to feline vaccine associated fibrosarcoma. Your cat's particular situation may vary. If you have any questions please ask us; we will be happy to answer them to the best of our ability.