A particularly frustrating tumor-type in feline veterinary oncology is the injection site sarcoma (ISS). Sarcomas are tumors of connective tissue and ISSs are a specific type of sarcoma arising at the site of a previous injection. The most common types of ISS in cats are fibrosarcomas, and the most common injections associated with the development of ISSs are vaccinations.
Other types of injections are also associated with tumor development, including microchips and injections of long-acting flea medications. In general, sarcomas are considered aggressive tumors; they are very locally invasive at the site of initial growth, and up to 25% metastasize to distant sites in the body; most commonly the lungs and local lymph nodes.
In the mid 1980s, veterinarians noted an increased frequency of sarcoma development in cats, with tumors occurring at locations commonly used for administering vaccines (the area between the shoulders, the lower back, and hind limbs). This increase in tumor formation at vaccine sites coincided with: 1) Introduction of laws legally requiring that cats be vaccinated against rabies virus, and 2) Increased use of killed vaccine products.
Subsequent studies showed the relationship between the development of tumors and vaccines was more than just a causal link, and there was an increased risk for tumor development when multiple vaccinations were given at the same site. These observations and study results led to the development of the Vaccine Associated Feline Sarcoma Task Force (VAFSTF) in 1996 as a means to more formally evaluate the association between vaccination and sarcoma development in cats.
ISSs are very rare tumors in cats, with reported frequencies between one in 1,000 to one in 10,000 cats vaccinated. Other studies report frequencies as one tumor developing per 1,000 vaccines administered. To put this in perspective, the average cat receives between 15 to 45 vaccines over the course of a typical 15-year lifespan (not including the more frequently administered kitten series).
ISSs are thought to arise as a result from an intense inflammation response to the actual physical injection itself, or to substances within the vaccine known as adjuvants. Adjuvants are materials added to vaccinations, serving to "hold" the vaccine within the skin for a period of time after the vaccine is administered, allowing the immune system to be properly stimulated. Adjuvants can also directly stimulate the immune system. We also know certain cats are genetically predisposed to forming ISSs.
Since sarcoma development is so rare, the current belief is that tumors develop from a combination of a particular cat being genetically predisposed to the tumor, along with the stimulation caused by the actual vaccine. Tumors can develop anywhere from 4 weeks to 10 years or more after receiving a vaccine.
Cats will commonly develop a lump at the site of a vaccination, typically caused by inflammation and local immune stimulation. These lumps are typically benign and will resolve spontaneously a few weeks after they are noticed. It is recommended to pursue a biopsy if 1) the lump is still present 3 months from the time of vaccination, 2) the lump is greater than 2 cm in diameter (approximately 1 inch) regardless of the duration of time since vaccination, or 3 the lump increases in size over the course of one month since the time it was discovered. Additionally, there are specific guidelines regarding where vaccines should be administered: rabies vaccines should be given as far down on the right hind limb as possible, feline leukemia vaccines should be given as far down on the left hind limb as possible, and all other vaccines should be given as far down on the right front leg as possible. Vaccinations should never be given between the shoulder blades.
If your veterinarian has a suspicion that your cat may have an ISS, the next step is to perform a biopsy of the lesion. I do not recommend aggressive surgery to remove the lump without attempting to obtain a biopsy first, as the best chance for successful treatment involves a very carefully planned and aggressive first surgery. Typically this requires referral to a specialty hospital, where pre-operative imaging with MRI or CT scan can be performed in order to accurately determine the extensiveness of the tumor prior to resection.
Surgery is the principal method of treatment for ISSs. However, sarcomas tend to grow and invade into underlying tissues and complete excision is a challenge. Up to 60 percent of tumors recur, often within the first 6 months after surgery.
Radiation therapy is typically recommended (either before or after surgery) as a means to “sterilize” the tumor’s edges and to decrease the frequency of recurrence. Chemotherapy is often recommended following surgery and/or radiation therapy as a means to prevent or delay spread of the tumor from the primary site to other sites in the body. I strongly urge cat owners to consider consultation with a veterinary oncologist if a diagnosis of ISS is suspected so all options can be discussed prior to pursuing definitive treatment options.
The diagnosis of an ISS is a particularly devastating one to me. Pet owners elect to vaccinate their cats to help them live longer, healthier lives, and when a tumor arises at the site of a previous vaccine they may feel guilty for "causing" their pets’ cancer.
It is important to remember that veterinarians recommend vaccinating cats as a means to protect them from deadly diseases that occur far more commonly than tumors do. You should discuss your cat’s vaccination plan carefully with your veterinarian. Some vaccines may be less likely to cause a tumor than others, and some can be administered using specialized injectors that disperse vaccines within a wider area of the skin or muscle, providing an alternative to traditional syringe and needle vaccines.
Together you and your veterinarian can decide what the best options are for your pet, with the goal of having him or her live a long and happy life as your companion.
Dr. Joanne Intile