INJECTION SITE SARCOMA
Back in the 1990's, researchers began to take note of locally aggressive tumors developing in the feline scruff of the neck, where vaccinations and therapeutic injections are typically given. The idea that vaccination (and potentially other types of injections) might occasionally cause cancer began to circulate in the feline medical community. By 1997, a task force had assembled to study the situation and make recommendations. We have come a long way since this time and will present to you what you should know about the Feline Injection Site Sarcoma (FISS) situation.
WHAT IS AN INJECTION SITE SARCOMA AND WHY DO WE THINK VACCINATION MIGHT CAUSE IT?
Soft tissue sarcomas have been recognized as difficult, deeply rooted tumors of the cat for a very long time, long before injections were ever involved. There are many types of soft tissue sarcomas with the fibrosarcoma being one of the most common. The fibrosarcoma is a tumor that does not usually spread throughout the body in the way we usually think of cancer; instead, it digs in deeply and widely in a localized area. After surgical removal, it is notorious for recurring even more aggressively than before. Distant spread does occur but much later in the course of the disease.
Fibrosarcomas can result spontaneously in either dogs or cats, as can any cancer, or they can be virally induced in cats via the feline sarcoma virus. While spontaneous and viral fibrosarcomas have been described for decades, the potential for vaccination to lead to the formation of these tumors is a relatively new concern. The idea that cats could develop a sarcoma associated with vaccination began to receive attention when some of the following facts emerged:
Reports of sarcomas stemming from other injections followed and it is now accepted that chronic inflammation concentrated in a small area is the root of the feline injection site sarcoma. Killed virus vaccine especially creates the exact scenario that can create a sarcoma in an individual with the right predisposition for it.
Despite the extremely low incidence of this problem, the problem is still very serious and the veterinary profession has responded with numerous studies on how vaccines might be related to tumor formation, why is the incidence so low given how many vaccines are given to cats annually, what can be done for prevention.
SO HOW OFTEN DOES THIS ACTUALLY HAPPEN?
Happily, not very often. Sarcoma incidence is extremely rare with different studies reporting an incidence between one in 1000 to one in 10,000 vaccines administered. To put this in perspective, using the traditional vaccination guidelines a cat would receive three vaccines annually during his or her fifteen year lifespan for a total of 45 vaccines, not including the initial kitten series. In other words, a typical cat would receive 45 vaccines in its lifetime and a sarcoma erupts in 1000-10,000 vaccines. And now that we have some prevention strategies, the incidence is likely to be even less.
It can take as short a time as two months and as long as ten years to develop a vaccine site tumor but most appear within 4 years of the offending injection. While most (80%) of the tumors that develop are fibrosarcomas, other types of tumors that can develop through this phenomenon include:
(All are tumors of either muscle, bone, cartilage or fat)
For a summary of the 2020 vaccinations guidelines for cats from the American Association of Feline Practitioners visit:
GUIDELINES FOR VACCINE-ASSOCIATED SARCOMA PREVENTION
Avoid unnecessary vaccination
Be wary of vaccination recommendations that encourage you to vaccinate for every possible disease. Recommendations are highly regional and individualized according to the philosophy of the animal hospital you are visiting and every veterinarian may have a different philosophy but the guidelines developed by the American Veterinary Medical Association, American Association of Feline Practitioners, American Animal Hospital Association, and Vaccine Associated Sarcoma Task Force are a good place to start (see link above).
The 3-2-1 Rule
Lumps commonly form in the weeks following vaccination due to the immune stimulation and inflammation centered on this area. These lumps are usually normal and do not represent fibrosarcomas (which generally take years to develop, not weeks). If your cat develops one of these lumps under the skin (they are usually noticed by owners 3-4 weeks after vaccination), the lump may be left alone to resolve naturally. If the lump is still present 3 months from the time of vaccination, it should be removed and biopsied. Any lumps greater than 2 cm in diameter (approximately one inch) should be removed no matter how long a time has past since vaccination. Also, any lump should be biopsied if it is felt to be getting larger rather than smaller one month after its discovery. ("3-2-1" refers to 3 months post vaccine, 2 cm in size, growing bigger after 1 month.)
Sometimes one such lump will break open. This usually means an infection is present rather than that a fibrosarcoma has developed. Your veterinarian should be informed of this occurrence and the pet should return for therapy.
TREATMENT FOR VACCINE ASSOCIATED FIBROSARCOMAS
Injection site sarcomas can be thought of as being similar to an octopus in structure. There is the "body" that you see but there are also far-reaching "tentacles" that cannot be seen extending far from the visible body of the tumor. For surgery to be successful, a very broad excision is needed and it is necessary to first determine how far the tumor has spread. Chest radiographs are vital for staging and CT scanning, if available, is very helpful in detecting the extent of the invisible tentacles. If CT scanning is not available, 5 cm (approximately 2 inches) should be removed around the visible portion of the tumor in all directions including any bones. Basic blood work including a feline leukemia virus test is needed to fully assess the cat for treatment.
If it is possible to remove 2 inches in all directions around the visible tumor (and consider for a moment how large a volume of tissue this is), the median survival time is two and a half years. The problem is that, unless the sarcoma is on a limb that can be amputated, it will not be possible to remove that much of the cat's body. If the complete tumor is not removed, it is almost certainly going to grow back. This is where radiation therapy can help a great deal. Whether this should be done before the surgery or after remains controversial and depends on the tumor grade and location. Radiation before surgery can shrink the tumor so as to make surgery more effective. Radiation after surgery can "clean up" bits of tumor not removed in surgery but must be delayed until the surgery site has healed and delay can be a problem. Radiation can also be used for "palliation" which means the goal is slowing the tumor growth rather than curing it. Fewer radiation side effects and less expense is associated with palliative treatment.
Chemotherapy is sometimes used in conjunction with radiation and surgery depending on the grade of the tumor and its location. Expect a referral to an oncologist to be needed to get the best and most up to date treatment plan.
Our goal in creating this web site is to create awareness of an emerging problem in veterinary medicine, not to elicit panic. We believe that intelligent decisions about pet ownership require information and education.
To review our hospital's current vaccination recommendations click here.
Page last updated: 7/27/2021