Injection Site Sarcoma in Cats: Lumps After Vaccines and What to Do
It is a worrying idea: a lump growing where your cat was vaccinated. The good news is that these tumors are rare. The more important news is that they can be dealt with better if found early. This guide explains the signs and the plan.
What is it?
An injection site sarcoma, also called a feline vaccine-associated sarcoma, is a tumor of connective tissue cells, often a fibrosarcoma, that arises at a place where an injection was given. It was first recognized in the 1990s. Chronic inflammation at the site is thought to trigger abnormal growth in a small number of cats. It is linked with vaccines, especially those that cause more inflammation, and also with other injections, such as long-acting drugs.
How common?
Estimates vary, but injection site sarcomas occur in perhaps one to a few cats per ten thousand vaccinated. The risk is small compared with the risk of the diseases that vaccines prevent. See cat vaccination schedule.
Signs
- A firm lump under the skin at a former injection site, such as the shoulder blades, back, thigh or flank.
- Growth over weeks or months.
- The lump may be painless, or may become painful, ulcerated, or fixed to deeper tissue.
- Some lumps are small, and some spread out like a flat plate.
Not all lumps are sarcomas. Many are small, harmless inflammatory nodules that fade.
The 3-2-1 rule
Vets suggest taking a sample if a lump:
- 3: is still present three months after the injection.
- 2: is larger than 2 centimeters in diameter.
- 1: is growing, or increasing in size, one month after the injection.
Write down the date of each vaccine, and where it was given. See cat vaccine reactions.
Diagnosis
The vet may take a needle sample or biopsy, and then run bloodwork, chest X-rays, and CT or MRI, to plan surgery. CT scans help show how far the tumor has spread under the skin.
Treatment
- Aggressive surgery. The main treatment is a wide excision, removing the tumor along with a margin of several centimeters of healthy tissue, and the layer of tissue below, which can mean removing muscle, ribs or part of a limb. A surgical oncologist or experienced surgeon gives the best chance of clean margins. The first surgery is the best chance.
- Radiation therapy, before or after surgery, to reduce regrowth.
- Chemotherapy, in some cases.
- Pain control and supportive care.
Even with treatment, these tumors often come back. Survival varies from months to years. Talk to your vet about goals. See cancer in cats.
Reducing the risk
- Use recommended injection sites. Many guidelines advise giving vaccines low on the limbs or at the tail, where a tumor can be treated by amputation. Different vaccines go in different legs.
- Use non-adjuvanted or recombinant vaccines where appropriate, as your vet advises.
- Only give vaccines the cat needs, following a risk-based schedule.
- Keep records.
- Check injection sites for a few months after.
Do not skip vaccines
The risk of sarcoma is real, but small, while the risk from rabies, panleukopenia and other diseases can be deadly. Talk to your vet to tailor a plan.
What to do if you find a lump
Do not wait. Note the date, location and size, take a photo and call your vet. Early biopsy matters.
Why early detection changes everything
A small tumor is far easier to remove with wide margins than a large one. A lump of 2 centimeters may need a modest operation, while one of 5 centimeters may need removal of ribs or a leg. For this reason, owners should check the vaccination sites every few weeks after injections, and report any lump that persists. Even a short delay can reduce the chance of cure. Cats with recurrent tumors usually have shorter survival.
What imaging adds
A CT scan reveals the tumor's shape, how deep it goes and whether it touches bone or muscle. That allows the surgeon to plan the margins, and the radiation team to design treatment. Chest X-rays or a CT of the chest check for spread, which is uncommon at diagnosis but possible.
Radiation and chemotherapy
Radiation, given in several sessions at a specialty center, may be used before surgery to shrink a tumor, or after surgery to kill remaining cells. Chemotherapy has a modest effect and is used in some cases, particularly if the tumor has spread. Your oncologist will explain options and expected benefit.
Making decisions
An injection site sarcoma diagnosis is hard. Ask about expected survival with and without treatment, the cost, side effects and quality of life. Some owners choose full treatment, others choose comfort care. There is no wrong choice if it is made with your cat's welfare in mind.
When to see a vet
See a vet for any lump that persists or grows after an injection. Use the US vet directory to find a clinic.
Frequently asked questions
How common are injection site sarcomas?
Rare, in roughly 1 in several thousand vaccinated cats.
What is the 3-2-1 rule?
Biopsy lumps that persist three months, exceed 2 cm or grow one month after injection.
Should I stop vaccinating?
Not usually, since diseases are far more common, so talk to your vet about risk.
Can it be cured?
Early, wide surgery can sometimes control it, but recurrence is common.
Sources and further reading
Health information changes as research does. These organizations publish guidance worth reading alongside our articles.
- American Veterinary Medical Association: pet owner resourcesPolicy-level guidance from the national veterinary association.
- CDC Healthy Pets, Healthy PeoplePublic-health advice on pets, zoonotic disease and safe handling.
- American Animal Hospital AssociationAccreditation body and practice guidelines for companion-animal hospitals.
Tailwisp is independent and written under our editorial policy. Learn who we are, or browse all cat guides.
This article is general educational information, not veterinary advice. Every cat is different. If your cat is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.