An injection-site sarcoma is a malignant tumor that grows under the skin of cats, in places where vaccines or other injections are given. Around 1990, vets in the USA first reported more tumors appearing at injection sites in cats [1]. At first they were called vaccine-associated sarcomas. When other injected products turned out to be linked as well, the name changed to feline injection-site sarcoma [1].
Keep the risk in proportion. These tumors are rare [2]. European cat experts say vaccination is an essential protection for cats and this risk is no reason to stop it [1]. Do not change your cat’s vaccines on your own; talk to your vet about the vaccination plan.
See your vet about any lump or hard patch where your cat has had an injection.
For other lumps on cats, see lumps and bumps on cats. For the related tumors in dogs, see soft tissue sarcoma in dogs.
Other names
You may also see the older name vaccine-associated sarcoma [1]. Most of these tumors are fibrosarcomas [1], tumors of the cells that make connective tissue. They have their own look under the microscope, with signs of inflammation and dead tissue inside the tumor [1][2].
What causes them
Nobody knows the exact cause yet [1][2]. The idea most experts accept is that long-lasting inflammation where something was injected can, in a few cats, push the local cells to turn into a tumor [1][2].
- Vaccines: studies link these tumors with vaccination sites, and the rise in cases was first tied to wider use of certain vaccines [1][2].
- Adjuvants: adjuvants are ingredients added to some vaccines to strengthen the immune response. Traces of them have been found inside some of these tumors, which is one reason they are suspected [1][2].
- Other injections: some injected medicines, and rarely a microchip or a buried stitch, have also been linked to these tumors [1][2].
Where they grow and what they look like
They grow in places where cats usually get injections: between the shoulder blades, on the side of the chest or belly, in the lower back and in the back legs [1]. Most sit under the skin, but some grow in the muscle [1]. A tumor can show up months or even a few years after the injection [1].
The lump usually feels firm and can look well outlined, but it is often stuck to the tissue below and does not move freely [1]. These tumors grow into nearby muscle and are larger than they feel from the outside, which is why vets often use scans before surgery [1][2].
How vets diagnose them
A lump in one of these places is not enough to name it. Your vet will take a sample of the lump, or remove it, and have it checked under a microscope [1]. A needle sample does not always give enough cells, so a larger tissue sample is often needed [1]. Before surgery, a CT or MRI scan shows how far the tumor reaches, and chest X-rays check whether it has spread to the lungs [1][2].
European cat experts say any lump at an injection site that does not go away or keeps growing should be sampled or removed and checked under a microscope [1].
Treatment
Your vet will advise on treatment and may refer your cat to a veterinary cancer specialist. Good teamwork between the family vet, the specialist, the surgeon and the pathologist matters for these cats [2].
- Surgery: the main treatment is wide and deep surgery that takes the tumor out with a broad border of healthy tissue around it [2]. A smaller operation that removes only the lump rarely cures it and often lets it grow back [2].
- Radiation and chemotherapy: radiation is often added before or after surgery [2]. Chemotherapy may be part of a combined plan, but on its own it is not the answer, and how much it helps is still debated [2].
At home, as directed by a vet:
- note the place and date of each injection your cat gets, so you can tell your vet;
- check the injection sites for swelling or lumps [1];
- go back for every recheck.
Vaccines and prevention
Vets have changed how they vaccinate cats because of these tumors [1]. The European guidance rests on three ideas [1]:
- give injections in places where a tumor, if one ever grew, would be easiest to remove completely, such as low on a leg;
- avoid injecting products that irritate the tissue; and
- make a vaccination plan for each cat [1].
Vets are also asked to write down where each vaccine was given [1]. Your vet can recommend the vaccines and schedule that suit your cat’s age, health and lifestyle.
Outlook
These tumors behave more aggressively than sarcomas in other places [1]. They spread into nearby tissue and often come back after surgery, which makes treatment hard [1][2]. Sometimes they spread to other organs, most often the lungs [1][2]. The outlook depends mostly on where the tumor is and whether surgery can remove all of it with clean edges [1][2]. Adding radiation or other treatment to surgery can improve it [1].
These are statements about groups of cats. They cannot predict how one cat will do. Your vet can explain what this means for your cat.
When to see your vet
See your vet if your cat has:
- a lump or hard patch between the shoulder blades, on the side of the body, on the back or on the back legs [1];
- a lump at an injection site that stays or keeps growing [1];
- a lump that comes back after it was removed [2].
FAQ
Should I stop vaccinating my cat?
No, not on your own. Experts say vaccination protects cats and should not be stopped because of this tumor [1]. Your vet can make a vaccination plan that fits your cat [1].
Is every lump where my cat had a vaccine a sarcoma?
No. A small swelling can follow an injection, and only a tissue test can tell what a lump is [1]. Show any lump at an injection site to your vet.
Can it come back after surgery?
Yes. These tumors often grow back, especially after smaller operations [2]. Your vet will plan rechecks.
The bottom line
Injection-site sarcomas are rare malignant tumors that can grow where cats get vaccines or other injections, often between the shoulder blades, on the side of the body or on the back legs. Vaccination still protects cats and should not be stopped because of them. A tissue sample confirms the diagnosis. Treatment is wide surgery, often with radiation, and the tumors often come back. Talk to your vet about any lump at an injection site and about your cat’s vaccination plan.
This article is for general information only and does not replace advice from your veterinarian. If your cat has a lump where it had an injection, see your vet.
Sources
- Hartmann K, Egberink H, Möstl K, et al. Feline Injection-Site Sarcoma and Other Adverse Reactions to Vaccination in Cats. Viruses, 2023 (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC10459272/
- Zabielska-Koczywąs K, Wojtalewicz A, Lechowski R. Current knowledge on feline injection-site sarcoma treatment. Acta Veterinaria Scandinavica, 2017 (CC BY 4.0). https://pmc.ncbi.nlm.nih.gov/articles/PMC5513368/