Cat Vaccine Injection Site Sarcoma: Signs and Risks

Name: Cat Vaccine Injection Site Sarcoma: Signs and Risks
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

Cat vaccine induced sarcoma is a rare cancer that can form at an injection site months to years after a vaccine. Most lumps that appear after vaccination are harmless and fade on their own, but a lump that keeps growing or lingers past a few months needs a veterinarian's eyes. Knowing what to watch for helps you act early without skipping the vaccines your cat needs.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

Feline injection-site sarcoma is uncommon. It is a recognized long-term risk of vaccination, but the risk is small compared with the protection vaccines provide [1].
Any lump at an injection site should be measured and tracked. A lump that is still growing one month later, or still present three months later, needs veterinary evaluation.
Vaccine site rotation (giving shots in different locations each time) is a standard risk-reduction practice, so ask your veterinarian where each injection was given.
Sarcomas are treated mainly with wide surgical removal, sometimes combined with radiation or other therapies [2].
Do not skip core vaccines out of fear. Your veterinarian can tailor a schedule to your cat's real exposure risk [1].

What Is a Cat Vaccine Induced Sarcoma?

A feline injection-site sarcoma (FISS) is a tumor that develops in the tissue under the skin where an injection was given. It is a sarcoma, which means it arises from connective tissue such as fibrous tissue or muscle instead of from the skin surface itself.

The link is not limited to vaccines. Any injection that causes local inflammation can potentially trigger this type of tumor, including antibiotics, anti-inflammatory drugs, and even sutures placed during surgery [2]. That is why many veterinarians now use the broader term "feline injection-site sarcoma" instead of tying it only to vaccines.

These tumors tend to be locally aggressive. They can grow into surrounding muscle and fat, and they can be difficult to remove completely because they send microscopic tendrils beyond the visible lump [3]. They are not generally described as spreading widely through the body the way some other cancers do, but local recurrence after surgery is a real concern.

Signs Owners Notice

The most common sign is a lump you can feel under the skin. It may appear at the back of the neck, between the shoulder blades, on a flank, or on a limb, depending on where the injection was given. In one review of cases, sarcomas developed in the thoracic region in about 24 percent of cats, the flank in about 21 percent, and the interscapular area (between the shoulder blades) in about 15 percent [4].

What a Suspicious Lump Looks Like

It is still growing a month after the injection.
It is larger than about 2 centimeters (roughly the size of a pea to a small marble) at any point.
It is still there three months after the injection.
It feels firm or fixed to deeper tissue instead of loose and movable.
It reappears after a previous lump was removed.

A lump that is soft, small, and shrinking is more likely a normal post-vaccine reaction. A lump that does the opposite deserves a closer look.

Other Signs

As a sarcoma grows, it can cause discomfort or interfere with movement. A tumor near a limb may cause limping. A tumor near the spine or pelvis can press on nerves and lead to weakness, trouble using the back legs, or loss of bladder and bowel control [3]. These are late signs, and they mean the tumor has had time to grow.

Causes and Risk Factors

Inflammation at the injection site is thought to be the trigger. The immune system responds to whatever was injected, and in a small number of cats that response goes wrong and cells begin dividing out of control [2].

What Raises the Risk

Repeated injections at the same spot. This is why site rotation matters.
Certain vaccines and adjuvants have historically been linked to higher risk, though the exact contribution is still debated.
Individual susceptibility. Some cats seem more prone to this reaction than others, and the reason is not fully understood.

What Does Not Cause It

A sarcoma is not an infection, and it is not contagious to other cats or to people. It is not caused by poor vaccine handling on your veterinarian's part.

Age patterns vary. In one case review, the average age at diagnosis was about 12 years, with a range from 5 to 17 years [4]. That does not mean young cats cannot develop one, only that it is more often seen in older cats.

How Often Does It Actually Happen?

This is the question most owners ask, and the honest answer is that the exact rate is hard to pin down. Published estimates vary widely, which has made it difficult for veterinarians to give owners a single reliable number [5].

What is clear is that feline injection-site sarcoma is considered a rare adverse event [5]. For context, millions of vaccine doses are given to cats every year in the United States, and the number of sarcomas diagnosed is small by comparison.

The takeaway is not that the risk is zero. It is that the risk is low enough that most veterinary organizations still recommend core vaccines for cats, while also recommending steps to reduce injection-site risk [1].

How It Is Diagnosed

Your veterinarian starts with a physical exam and a careful history. They will want to know when the injection was given, what was injected, and where. Measuring the lump and recording the size is the first step, because change over time is one of the most useful clues.

Tests Your Veterinarian May Recommend

Fine needle aspiration. A thin needle collects cells from the lump. This can suggest a sarcoma, but it cannot always confirm one.
Biopsy. A tissue sample gives a definitive diagnosis and helps identify the tumor type. This is the standard way to confirm a sarcoma.
Imaging. X-rays, ultrasound, CT, or MRI can show how deep the tumor goes and whether it is near nerves, bone, or organs [3].

A biopsy is important because not every lump is a sarcoma. Some are benign growths, cysts, or simple inflammatory reactions. Your veterinarian may also consider other tumor types that can appear in similar locations, such as nerve sheath tumors [6].

Treatment Options

Surgery is the main treatment. The goal is a wide excision, meaning the surgeon removes the tumor plus a margin of normal-looking tissue around it. Clean margins give the best chance of preventing regrowth [2].

When Margins Are Not Clean

If the pathologist finds tumor cells at the edge of the removed tissue, additional treatment may be recommended. Options include:

Radiation therapy, given after the surgical site has healed [2].
Electrochemotherapy, which uses electrical pulses to help a drug enter tumor cells in the resection bed [2].
A second surgery to remove more tissue, when the location allows it.

When the Tumor Is in a Difficult Spot

Some sarcomas sit next to structures that cannot be removed, such as major nerves or the spinal cord. In those cases, surgery may be limited, and the focus shifts to controlling the tumor and preserving quality of life [3]. In one reported case, a sarcoma involving the pelvic limb region was treated with surgery and radiation, and the cat later developed a spinal lesion about nine months after finishing radiation [3].

What About Chemotherapy?

Chemotherapy is used more often for lymphoma and other cancers that spread through the body [7]. For injection-site sarcomas, which are primarily local tumors, surgery and radiation are usually the first-line approaches. Your veterinarian will decide whether chemotherapy has a role in your cat's specific case.

Situation	Typical Next Step
Small lump, shrinking, less than 1 month old	Monitor at home, recheck if it changes
Lump still present at 3 months	Veterinary exam and likely aspiration or biopsy
Confirmed sarcoma, accessible location	Wide surgical excision
Confirmed sarcoma, incomplete margins	Radiation, electrochemotherapy, or repeat surgery
Tumor near nerves or spinal cord	Advanced imaging, possible limited surgery, palliative care

Recovery, Outlook, and Long-Term Management

Recovery depends heavily on where the tumor was and how completely it was removed. Cats that have clean margins after surgery tend to do well, though they still need regular rechecks because recurrence is possible.

What Follow-Up Looks Like

Recheck exams every few months for the first year or two.
Imaging if your veterinarian is concerned about regrowth.
Watching for new lumps anywhere on the body, not only at the original site.

Living With a Diagnosis

If the tumor cannot be fully removed, your veterinarian will focus on comfort. That may mean pain management, wound care, and monitoring for signs that the tumor is pressing on something important. Cats can live with a sarcoma for a meaningful amount of time with good supportive care, though the outlook varies case by case.

One thing that helps: keep a simple record of every injection your cat receives, including the date, the product, and the location. If a lump ever appears, you will have the information your veterinarian needs.

Prevention and Risk Reduction

You cannot eliminate the risk entirely, but you can lower it.

Vaccine Site Rotation

This is the single most practical step. Instead of giving every injection in the same spot between the shoulder blades, veterinarians give vaccines in different locations, often a different limb or a different side of the body each time. This does two things. It reduces repeated inflammation in one area, and it makes it easier to identify which injection might be linked to a lump if one appears.

Ask your veterinarian to tell you exactly where each injection was given, and write it down.

Tailored Vaccine Schedules

Not every cat needs every vaccine every year. Decisions about which vaccines to give an adult cat and how often depend on the cat's exposure risk, the duration of protection of each vaccine, and the risk of the cat passing diseases to people [1]. Indoor cats with no exposure to other cats may need fewer vaccines than cats that go outdoors or live in multi-cat homes.

Core vaccines still matter. Feline panleukopenia, herpesvirus, and calicivirus are serious diseases, and the FVRCP vaccine protects against all three. For cats at risk of feline leukemia, the FeLV vaccine is worth discussing with your veterinarian. You can read more about which vaccines are considered essential in our guide to core cat vaccines.

Other Steps

Combine injections when possible so your cat gets fewer needle sticks.
Use the smallest vaccine volume that provides protection.
Report any lump to your veterinarian right away, even if it seems minor.

If your cat has had a reaction to a vaccine before, tell your veterinarian before the next appointment. Mild reactions like sleepiness or a small bump are different from a serious reaction like facial swelling or trouble breathing. Our article on vaccine reactions in dogs and cats explains how to tell them apart.

Limitations and When to Contact a Veterinarian

No article can tell you whether your cat's lump is a sarcoma. That requires an exam, and often a biopsy. Your veterinarian is the only one who can make that call for your cat.

Contact Your Veterinarian the Same Day If:

Your cat has a lump at an injection site that is growing quickly.
The lump is painful when touched, or your cat flinches or pulls away.
Your cat is limping on the limb where the injection was given.

Go to an Emergency Clinic Now If:

Your cat has trouble breathing, facial swelling, hives, or collapse after a vaccine.
Your cat cannot urinate or has lost control of bladder or bowels.
Your cat suddenly cannot use the back legs.

Schedule a Routine Visit If:

A lump is still present three months after vaccination.
A lump is larger than about 2 centimeters.
You are not sure whether a lump is new or old.

Frequently Asked Questions

Should I skip vaccines because of sarcoma risk?

No. The risk of sarcoma is low, and the diseases vaccines prevent can be severe or fatal. Your veterinarian can adjust the schedule and the number of injections based on your cat's lifestyle and age [1].

How long after a vaccine can a sarcoma appear?

It can take months to years. That long delay is one reason the link was not recognized for a long time. If a lump appears long after an injection, it still deserves evaluation.

Can a benign lump turn into a sarcoma?

A simple post-vaccine lump does not usually transform into cancer. They are different processes. The concern is a lump that behaves abnormally from the start, growing steadily instead of shrinking.

Does the type of vaccine matter?

Some vaccines and adjuvants have been linked to higher risk, but the evidence is not clear enough to single out one product. Site rotation and limiting unnecessary injections are the strategies most veterinarians rely on.

Will my cat need chemotherapy?

Usually not. Injection-site sarcomas are mainly local tumors, so surgery and sometimes radiation are the primary treatments [2]. Chemotherapy is reserved for specific situations your veterinarian will discuss with you.

References

Feline Vaccines: Benefits and Risks. Cornell University College of Veterinary Medicine.
Feline Injection-Site Sarcoma Treated with Surgery, Orthovoltage Radiotherapy, and Electrochemotherapy. A Retrospective Series of 72 Cases. Bioelectricity, 2025.
Suspected recurrent sarcoma resembling feline injection-site sarcoma with spinal cord invasion in a cat. JFMS open reports, 2026.
Clonality assessment and feline leukemia virus protein expression in injection-site sarcoma-associated lymphocytic infiltrates. Veterinary pathology, 2026.
The incidence of vaccine-associated sarcomas in cats: a systematic review and meta-analysis. The Journal of small animal practice, 2026.
Malignant nerve sheath tumor infiltrating the epididymis of an inguinal cryptorchid testis in a domestic shorthair cat. JFMS open reports, 2026.
Immune System Tumors in Cats - Cat Owners. Merck Veterinary Manual.