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.

Photo: morguefile.com

20 Comments on Injection-site Sarcomas in Cats

  1. To Heidi Henson,
    Do you know if Emma’s biopsy was sent to a pathologist for reading? I went through this earlier this year with my cat, Ellie. The vet’s office gave me the option of having the needle biopsy slides sent to a local hospital. The cost was an extra $140 dollars but that was less than the cost of surgery and they were able to confirm the diagnosis. I’m sending love and positive energy to you and Emma, hoping she has a better outcome than my Ellie.

  2. My Emma developed a lump about 3 months ago on her left rear leg. It started flat and 1 1/2 ” big, and reduced to 1/2 inch lump. My vet is recommending the wedge biopsy, first. But they can’t check for clean margins while she is under, like they do for humans. Do you know anyone who does that? I’m tempted to have my cat x-rayed first to see if it has spread (if it even is sarcoma – the needle aspirate was inconclusive and vet says typically they don’t go from big to small – so not acting like a typical sarcoma). The surgery is $1000+ and it’s just for diagnosis, not even treatment. I am overwhelmed with the decision. Emma is 7 years old, healthy, happy, active. I know I won’t put her through all the treatments if prognosis is living up to 2 years post treatment. I am looking for advice and input. Please.

    • You may want to consult with a veterinary oncologist, Heidi. Sadly, as you probably already know, the prognosis for these types of tumors is poor.

  3. Last night I found a large lump on my cat’s back leg. She has long, very thick fur so it was not visible and I did not realize it was there until I felt it. I took her to the vet this morning and they did x- rays and a needle biopsy. They shaved her leg to get a good look and I was stunned by how large it is. I do not have results back yet but since it is where she received her rabies vaccination the vet said she thinks it may be injection site sarcoma. She is going to contact me Monday with biopsy results and we can go from there. I had indicated that I would be okay with removing the mass but don’t think I want to put her through chemo and radiation. She said that due to the size, removing the mass probably won’t be an option but that they would need to amputate her leg. From what I have been reading, even with amputation, chemo and radiation she would not be cured and would most likely not live more than another 1 or 2 years. I don’t think I want to put her through the stress of that treatment when there is no real chance of a cure. Right now, she is not showing any signs of illness. I know I need to wait for the biopsy results but can’t help thinking about all of this. I want to do what is best for her, not what is best for me. It seems like the treatment would not give her the quality of life she has now. My thought is to just make her as comfortable as I can, give her extra love and when the time comes that she is no longer enjoying her life I will help her end it peacefully. This seems like the kindest thing to do. She is only 3 years old and I am heart broken. Does anyone know how quickly this type of cancer progresses without treatment?

    • My heart goes out to you, Carol. As you know from your research, the prognosis for these cancers, even with treatment, is poor. I know this is a difficult decision, and that ultimately only you can make. You know your cat best. I believe that as long as you make her quality of life the top priority, you will make the right decision for her, even if it’s a heartbreaking one for you. If you elect to keep her comfortable rather than choose aggressive treatment, this article may help: https://consciouscat.net/2016/02/08/hospice-for-cats/

      • Ellie’s diagnosis has been confirmed as injection site sarcoma, caused by the rabies vaccination she received a few months ago. Her vet is recommending amputation of her leg because it might give her a chance for recovery. I feel that given the size of the tumor and how quickly it has grown that it is too late. Right now she seems to be fine, eating well, still playing, running, jumping and does not appear to be in any pain. I really feel as if amputating her leg will be the beginning of the end. I just can’t do that knowing that she will probably then need radiation, which requires anesthesia. Then it will be chemo. She will not understand that I am trying to help. She will only know that I take her places, leave her there and bad things happen. She will become afraid of me. My other cat reacts badly to any change and is still shunning her. So in addition to losing her leg, she will also lose her bond with me and my other cat. I feel so bad because I am not doing everything possible, but I also feel there is no hope. I hope she will remain pain free for a long time because I don’t know how I am going to know when it is time to help her to peacefully leave this world. How do I handle the guilt I am feeling? What if I am making a mistake by not treating her? I just don’t know what to do.

    • Carol,
      I know I am being guided to this issue right now for I would never ever EVER have seen your post or go on this website! The energy that took down Notre Dame on April 15 was so powerful it got the best of my beloved cat Jasmine too! I miss her so and have been lost without her. However, it’s a happy kind of sad since she was 23. I adopted her in 1996 with her litter mate when they were just ten weeks old. She, unlike her brother George, was a sensitive kitty and had experienced three injection site tumors starting in her early years around the same age as your cat. He never did and lived to 18. But Jasmine was a trooper! I remember how scared I was thinking it was the end for her when she first developed the tumors. The first two times she had surgery to have the lump removed and they also cut into her muscles even though it wasn’t sarcoma. These were major surgeries where she needed the cone afterwards. The third time she developed a tumor, the new young doctor informed us of issues some cats have with live viruses versus killed viruses in the vaccines and eventually develop sarcoma. He put her on anti-inflammatories, which OMG reduced the lump completely in a few weeks!!!!! AND he recommended she never get the distemper or leukemia shots again since she clearly was one of those cats exhibiting sensitivities to the vaccines. You can imagine how bad I felt knowing she most likely could have avoided all the pain from 2 surgeries if I would have known about vaccine reactions in cats and if the other doctor would have suggested anti-inflammatory meds first. However, she didn’t know herself. My cat still received the rabies vaccine (which is required by law or most vets won’t see your pet).PLEASE, PLEASE try something less invasive like anti-inflammatories, even acupuncture or laser therapy before surgery, if it is not cancer. I wish you the best of luck and hope your cat is just one of those super-sensitive kitties like mine was and you are able to enjoy many more years with her.

      • Unfortunately, her biopsy confirmed that she does have injection site sarcoma. The tumor is too large for removal. Her vet is recommending amputation as her best chance. At this time she is doing well, I would never have known anything was wrong if I had not found the tumor. It was hidden under long, thick fur so I was not able to find it early. I did not know a vaccine could cause this so I did not know to monitor the site. I feel the kindest course of action is to provide extra love, pain meds when she develops the need for them and when her quality of life is gone she will be helped to peacefully cross over to the other side. I am completely devastated but do feel this is the best I can do for her. I am so glad your cat recovered but it does not seem to be possible for mine. I’m just hoping for the strength to see this through.

  4. When I was growing up, we had three cats, all brothers. Each of them developed sarcoma later in life (after 10 years) at the vaccine site. We did not treat it, but made their lives comfortable until the tumors were not manageable any longer and had them put down. There was about a 3-4 year gap between each of them. My cousin works with this kind of research and let us know back then what it most likely was, but more research is still yet to be done. Do they know if there is any genetic link or predisposition?

  5. I had a cat with VAS in the 1990s, and we went through the whole nightmare of surgery, chemo and radiation. She died in 1997, two years after diagnosis, at the age of 13. Another family member has a cat with VAS. He’s still okay a few years after aggressive treatment. VAS is far more common than anyone would suppose. I was following the “vaccinate every year” protocol back then. Now, 16 years on, I’ve made **very different decisions for our kitties.** Enough said.

  6. I’m very interested in this topic and am so glad to see that these organizations are all now advocating for less frequent vaccines. We stopped FeLV altogether in ours since they’re indoor only and not exposed to other cats. And I’m on the fence about FVRCP since I volunteer weekly at a shelter.

    I do practice extreme caution when I return home, changing clothes in the laundry room before entering the house, and then showering immediately. And the shoes I wear to the shelter are my shelter-only shoes and stay out in my car.

    But still…
    Maxwell had calici when we got him, which means he could shed the virus the rest of his life. Neither of the others have shown any evidence of it, and once we returned Max to good health, he’s shown no sign of any illness, calici or otherwise, since. So is FCRVP really necessary for an indoor adult cat?

    I’ve seen firsthand how devastating the virulent form of calici can be, when it shows up in a shelter, and I’m terrified of bringing something like that home.

    The vaccine issue is what my scientist husband would call a “multi-variable problem”.

  7. I am beyond annoyed at the change of VAS (Vaccine Associated Sarcoma) to “Injection site Sarcomas” I think it does an absolute injustice to the menace that is VAS.

    I have given many an injection to my diabetic cat over 7 years and she never had an issue.
    I know many diabetic cat owners who also have not had an issue.

    I know many owners of cats who have had “ISS”, and they were ALL VAS..

    Ollie had VAS, and I didn’t take it as seriously as I should have the first time around. I figured simply removing it would be enough. If I knew then what I knew now I think I would have chosen to have someone other than my vet preform the surgery to make sure they got excessive margins around the tumor. When his tumor came back the second time it started at the ends of where the original tumor was. VAS is insidious and invasive, and I will be constantly be talking about the dangers of vaccines.

    I still believe in the effectiveness of them, but I do fully believe we are over vaccinating our cats. Do you know that a 150lbs dog and a 2 lb kitten get the same vaccine.. same dose, same strength, same everything.

    Do you know that the AVMA has changed their vaccine protocol so that FELV and FVRCP should be given every three years after the first annual booster and not annually for life? Nothing changed in those vaccines either, and many vets are still boostering annually – be it for profit or because they aren’t up to date on the data, or because they fear they won’t ever see their cat clients if they do not recommend boosters every year..

    You are in charge of what medications go into your pet. You have every right to decline anything your vet recommends.. (Most communities rabies is required by law and if your pet ever bites anyone while not current on vaccines you could run into some legal issues)

    • Connie, I think it’s important that cat guardians understand that it’s not just vaccines that cause these tumors. There’s still much that is not known about fibrosarcomas, and more research is needed. The AVMA, AAHA, and AAFP all recommend less frequent vaccinations, and I think it’s a step in the right direction.

      I absolutely support less frequent vaccinations. Instead of vaccinating every three years, you may have the option to test vaccine titers. Rabies vaccines are, unfortunately, required by law, and until those laws change, the best a cat guardian can do is make sure that cats receive the non-adjuvanted vaccine.

      However, I think there is some danger in maligning all vaccines as bad. Vaccines prevent diseases that used to kill cats. As with so many things, it comes down to evaluating risk vs. benefit, and I believe education is key.

  8. I lost a cat to VAS when they were just starting to become a big problem in cats in the late 1990s. We had 1 cat come into the vet clinic who developed it and when I picked Taye up one day after coming home from work and found it, I knew what it was. It happened 3 years after Taye received her rabies vaccine and I had been doing reduced vaccines for awhile. Taye was my heart cat and with treatment options limited at the time, I was thankful to ahve 18 more months with her.

    • Dawn I’m interested to know what option you chose to treat your cat. My 15 yr old was just diagnosed with stage 1 cardio myopathy and stage 1 congestive heart failure and a vaccination site sarcoma on his hind leg that is 10cm in diameter. They said amputation is the only way to remove the cancer and the cardiologist said that doing that surgery is to risky with his heart disease. I’m between a rock and a hard place. Only reason I found any of this out is because the vet heard a gallop and performed an X-ray otherwise my boy was acting normally

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