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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

The Frustrating Vaccine Related Sarcoma

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



Image: Ilike / via Shutterstock

Comments  16

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  • Rarity
    12/05/2012 11:44am

    I once heard a veterinarian speak and say the incidence of ISSs was about 1 in 10,000 cats. An audience member asked how many ISSs the vet had treated and the response was (I think) 4 or 5.

    In my mind, I asked, "So you've vaccinated over 40,000 cats?"

    I'm glad the standard injection sites have been standardized and changed to no longer use the scruff, but I wonder if ISSs are much more common than the statistics indicate.

  • 12/06/2012 07:20pm

    I am a veterinarian, almost 8 years out of veterinary school.
    Let's assume that I work at a feline-only clinic 5 days per week, 8 working hours per day. 8 years out, I have finally earned 4 weeks of vacation or 20 working days off per year, so that means I work about 240 days per year. I see 3 appointments per hour, or 24 patients per day. Let's assume that about 2/3 of these appointments are for vaccines, so I give at least 1 vaccine to 16 patients daily, or about 3840 patients per year. Over the last 8 years, that means I've vaccinated about 30, 720 cats. In another 3 years, I will have vaccinated over 40,000 cats. I still consider myself a relatively "new" vet, so if the vet whom you heard speak has been practicing for even a few years longer than I have, then yes, s/he has vaccinated over 40,000 cats.

    VASs absolutely do exist.
    Rabies, feline herpesvirus, calicivirus, panleukopenia, and feline leukemia virus also absolutely do exist.

    In my own almost-8 years of practice, I have seen significantly more cases of leukemia/lymphoma, ulcerated and perforated eyeballs, painful oral and tongue ulcers, and even confirmed Rabies cases, all preventable with vaccination, than I have seen VASs.

    I think the author does make the point that you and your veterinarian need to consider the benefits vs risks of a particular vaccine, just as you would any medical treatment. I also think many of the people who are so quick to condemn vaccines have lost sight of what the vaccines' original intent was, that is to prevent preventable disease. This debate will likely go on for as long as will the debate over vaccinating children.

  • 12/07/2012 04:09pm

    I hope that you are one of the good vets who informs their clients of potential risks with any vaccine and who vaccinates as low on the leg as possible in case VAS develops. The problem most of us have is that our vets never told us about VAS as a possible side effect and we were unable to make an informed choice about our pet' vaccines.

    I do wish more vets would offer titer testing. Why is it that humans don't require annual revaccinations like cats and dogs?

  • not that rare
    12/05/2012 05:25pm

    I'm sorry, but a "1 in 1000" chance is NOT rare.. I don't think many of us humans would take that risk at those odds, especially since VAS is so often fatal. Also, why would you not include the kitten series vaccines? They are the exact same vaccines as given to adult cats (in the same dose and the same amount..) That is if that stat is even accurate. How many vets actually report incidents of VAS. I know my vet never told anyone but me - or if he did he didn't tell me that he did. I didn't follow through with it because I knew of the horrible record keeping from my previous vet, I could never prove which vaccine gave it to him nor which manufacturer was injected.

    I have read a lot of interesting stats on vaccines since my Ollie developed VAS. I refuse to call it ISS, because I had a diabetic cat for over 7 years and in her lifetime gave over 5,000 injections. I don't doubt that there has been sarcoma or two due to some other type of injection. I also wouldn't add the sarcomas due to micro-chipping to the ISS umbrella, because the thought that it is the microchip itself that causes the irritation, not the needle that injected it.

    I also find the stats on VAS on dogs quite interesting, in the fact that there is a higher incidents of it among smaller dogs vs larger dogs. So startling when you start to realize that the same amount of vaccine is given to a 2lb kitten as a 200lb dog.

    I find it abhorrent that most people aren't even aware of the side effects of vaccines. There are more issues besides VAS. When was the last time your vet actually told you of the issues? Did they mention to look for lethargy and soreness at the injection site? Did they tell you of all the side effects possible, or even offer the pamphlet that comes with it for you to read? or is it basically a 'here is a lollipop' type attitude and send you on your way?

  • 12/06/2012 04:17pm

    This is very good information. I have heard about the microchip issue, and did not know it was the microchip and not the injection that seems to be the reason for the swelling. Thanks.

  • Rare ???
    12/05/2012 08:49pm

    I must take issue with the comment that Vaccine Associated Sarcoma is "rare," as well as with the term ISS (Injection Site Sarcoma.)

    I belong to an on-line support group for folks dealing with VAS. Among the couple thousand cat owners who have passed through the group since I joined eleven years ago, we have had more than a dozen who've had two VAS cats, and another dozen or two more who had one officially diagnosed VAS kitty and who also lost a cat some years earlier to a cancer that, in retrospect, sure looked like VAS, despite the absence of an official diagnosis. Rare? I don't think so!

    Injection Site Sarcoma is a term that's been promoted by vaccine manufacturers and their apologists to spread the idea that other kinds of injections can trigger cancer. Of the couple thousand VAS cats that have passed through the VAS Support Group since I joined, we have had two (2) whose cancer was apparently triggered by injections other than vaccinations. Two!

    Oncologists believe that the VAS trigger is inflammation, which is an integral part of the immunization process. (Ref. the transcript of the VAS Task Force's Round Table Discussion; participants included most of the US's leading veterinary oncologists and immunologists.) All vaccines produce some degree of inflammation; they wouldn't work without it. Cats (and ferrets) have the misfortune to be especially susceptible to the carcinogenic effect of inflammation. That doesn't make VAS the vaccine manufacturer's "fault," but I wish more of them would put their efforts into producing vaccines that work with less inflammation (like Merial's PureVax) rather than trying to deflect our awareness of vaccines as a cancer trigger in cats.

  • Not rare
    12/06/2012 03:14am

    Visit these resources and determine for yourself
    Sign the petition https://www.change.org/petitions/veterinarians-fully-inform-us-before-vaccinating-our-dogs-and-cats
    Watch the video. http://www.youtube.com/watch?v=2nZ5m5uzzac
    Educate before you vaccinate

  • Why?
    12/06/2012 03:48am

    Just curious, I have posted my comments twice now and they have been deleted? Is this not an open forum to discuss this article? Why delete comments from someone who knows firsthand knowledge about this subject?

  • 12/06/2012 02:00pm

    I'm disappointed (though not surprised) that Pet MD is deleting your comments. It's really interesting that your comments would be erased because this is what they say at the bottom of the article:
    "All opinions are welcome on The Daily Vet. So bark away... I don't bite."
    You may not bite, but it seems you do censor.

  • 12/06/2012 04:20pm

    If all comments are welcome, none should be deleted. I think everyone should have as much information about this disease that can be offered.

  • Re:
    12/06/2012 04:55pm

    Hi "Stimpy."

    I am the comments moderator and I can assure you that your comments were not deleted. I only delete comments that are vicious or offensive in nature, that are self promoting in a way that is not appropriate on petMD, or that are spam (outright ad trolling). Based on what I read in the private e-mail you sent, if you had left the same comment on site I would not have deleted it because we do promote open discussions - even when they are dissenting points of view.

    Because I have no record on my side of the comments you attempted to post, I am guessing that one of the words on our "flag list" (that is, words that are flagged because they are frequently used in offensive or spammy ways) was included in the content of your comment. Unfortunately we do not have a high tech system that shows you which word is being flagged so that you have an opportunity to change it - the entire comment just disappears. This is frustrating for us as well as our readers and it is something we look forward to changing. This change is not high on the list of priorities for our site developers, however, so in the meantime what I can recommend to you and to all of the members of our community is to copy and paste your comment - especially if it is a long one - onto a separate document sheet on your desktop so that you will not have to rewrite it should it disappear. If the problem persists, you are welcome to write to me at [email protected] and I can look into why the comment will not post. We have done this before for members of our community.

    We do appreciate reasonable arguments from the different voices of our community, especially so for an issue that is emotionally wrought for some pet owners, such as this is. We also appreciate that you remain as a member of the petMD community even when you do disagree. As someone who also posts dissenting points of view on the web (when called for), let me reassure you that your comments are valuable to us.

  • Not rare!
    12/06/2012 01:57pm

    My small-town EX-vet has seen at least three cases of VAS and he still continues to vaccinate in the scruff!
    My cat, Chicken, developed VAS almost a year after her last vaccination. Turns out her vet had been vaccinating her with 3 year vaccines every year. I trusted him and I lost big time, but not nearly as much as Chicken lost. Veterinarians have a responsibility to be truthful with their clients and to use best practices. Luckily I have a new vet who shares my philosophy on vaccines, escpecially for indoor kitties.

    I wish people would quit painting this as a rare side effect. VAS should be presented to every client as a possible side effect BEFORE vaccines are given so that owners can make an informed decision.

    The term ISS is just a cop out that the vaccine manufacturers are using to try to deflect the blame from their deadly vaccines.

    Come see just how beautiful Chicken was:

  • Not as rare as you think
    12/06/2012 04:33pm

    I also lost a beautiful black kitty, Dixie, at age 5 1/2 from this horrible disease. I took her for state required rabies vaccines, and in a little over a year she was gone. The vaccine clinic did mention there would be possible side effects, and a "rare cancer" in a very brief disclaimer. If I had more information, I could have made a much more informed decisions.

    I also disagree with at least two points in this article. One of course trying to take the responsibility of the vaccination company away by callin this disease ISS. The other is not to count the kitten series, they are still vacinations.

    As you can see from the number of comments here, and the stories that are told, this is not as rare as people want us to believe.

  • How common in dogs?
    12/07/2012 12:43am

    Is this being seen more often in dogs as well? We lost a year-old whippet puppy to a sarcoma that showed up at her injection site. Although the vets who diagnosed her did not suggest it, I wonder if that's not what it was.

    And yes, it was devastating. She was the family's first dog, they had two children who were completely smitten with her, and it was just a horrible, horrible experience. It was two years before I could convince them to accept another (free) puppy into their family. As the dog's breeder I felt responsible, even though the vets assured me there was nothing I could have seen or done before the pup left home. I am happy to report that the mother and daughter came to pick up "Tryst" from me last week. As I told her goodbye, I whispered, "Please be healthy and live a LONG time!"

    Should puppies and dogs also receive vaccinations in a different site?

  • trust
    12/09/2012 12:40pm

    Firstly and honestly i know so little about the effects of any injection into my own body, and have to rely upon the knoweldge of the people who produce the various vacines that we all so gulibly take,having now had a dog for some 9 years and she a regular for her treatment
    i am becoming even more sceptical about the use of any drug administerd to my pet,
    Yes and i have seen parovirus outbreak which wiped out a complete litter of pups that my father had bred,70 years ago, a foolish money saving plan which proved even more expensive in the end,
    so how far do we as the general public go in finding all the facts about what may or may not happen.
    Penny my boxer was microchipped at 3 months old, did not show any side effects, now at 9 year old has run into all sorts of trouble,even though having been a regular vet visiter all her life,receving her yearly inoculations and check ups, now my mind is in turmoil having read the postings.
    who do i believe?
    ray greening

  • 12/12/2012 04:38am

    Just out of curiosity, have you ever seen an ISS associated with long-term insulin administration?

    My soon-to-be 12 y/o cat was diagnosed with diabetes at three, and I've often wondered if the thousands of scruff injections he's received over the years pose any significant risk in the ISS department.

    I would assume that the risk probably wouldn't be comparable to that of vaccines, as they're designed to stimulate an immune response, and one would assume that this is something you'd attempt to avoid when formulating insulin, but still... that's a lot of injections.

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