An injection-site sarcoma is a tumor of the connective tissues in the cat. These tumors are often called fibrosarcomas, and are most frequently located between the shoulder blades, in the hip region, and in the back legs. They are most often associated with inactive killed rabies or feline leukemia vaccines, or with multiple vaccines given at the same time, but they can also be caused by other injections such as steroids. They have even been associated with microchips. The incidence of these tumors ranges from 1 in 1000 to 1 in 10,000 cats. They can develop as quickly as 4 weeks or as late as 10 years post vaccination.
The first step toward diagnosis is a fine needle aspirate of the lump. Your cat’s veterinarian will insert a small needle directly into the tumor and extract cells. This is an inexpensive and minimally invasive test, but unfortunately, it is also not very accurate and can lead to a high rate of false negative results. In most cases, a surgical biopsy will be necessary for a definitive diagnosis. A complete blood count and blood chemistry, along with a urinalysis, will help determine whether cancer cells have spread elsewhere in the cat’s body. Your vet may also want to take radiographs and/or perform an ultraound to determine whether the cancer has spread.
Surgery is almost always the first step. Tumors will need to be excised with a wide margin, which means that the surgeon will also need to remove surrounding tissue, since these tumors tend to infiltrate surrounding tissue. Cats with “clean margins” tend to fare better, but these tumors have a high recurrence rate, and follow up treatment with radiation and/or chemotherapy are almost always recommended.
A full course of radiation is recommended to reduce the risk for local recurrence. Typically, radiation therapy is performed once a day for 4 weeks. Cats will need to be anesthesized for each treatment, and this may not be an option for all cats. Aside from the anesthetic risk, radiation therapy is generally well tolerated by cats. Median survival times for cats undergoing radiation following surgery range from 300 to 1300 days.
Chemotherapy may be recommended in lieu of, or in addition to, radiation, depending on the severity of the grade of the tumor. Most cats tolerate chemotherapy well.
Injection-site sarcomas are difficult to treat. Cats with tumors on the legs, where an amputation can be performed, tend to fare better than cats whose tumors are on the trunk of the body. Cats with recurring tumors have a poorer prognosis; the best time to treat these tumors aggressively with surgery, radiation and/or chemotherapy is the first time they occur.
Since not enough is know about what predisposes some cats to these tumors, it is difficult to determine how to prevent them. Discuss your cat’s vaccination requirements with your veterinarian and avoid over-vaccinating your cat. Use titer testing rather than having your cat revaccinated whenever possible. Make sure your vet uses non-adjuvanted vaccines.