Injection Site Sarcoma In Cats

Vaccines are a crucial part of maintaining your cat’s overall health.

Core vaccines not only protect our feline friends against infectious disease, but some are even required in specific situations.

While the benefits of vaccines far outweigh the potential risks involved, some unlucky cats will suffer from rare adverse reactions.

Injection-site sarcomas are one of the potential complications that come along with feline vaccinations, though the condition is still considered very uncommon.

So what is an injection-site sarcoma, and how likely is it for your cat to develop this disease?

In this article we will discuss the details surrounding injection-site sarcomas in our furry friends, and help you better understand what this diagnosis means for your cat going forward.

Injection Site Sarcoma In Cats

What Is A Sarcoma In Cats?

Before we discuss the details of sarcomas and their relation to vaccines, it’s important to understand what a sarcoma is in the first place.

A sarcoma is a cancer that develops within the tissue of our feline friends, impacting tissue such as the skin, cartilage, muscle, blood vessels, and peritoneum.

Not only can sarcomas impact the mesenchymal tissues, but they can also spread to surrounding areas when left to progress.

Sarcomas are considered locally aggressive, and require immediate intervention to decrease the chance of metastasis.

What Is An Injection Site Sarcoma In Cats?

Injection-site sarcomas are cancerous masses that occur in locations where vaccines were previously administered.

These masses typically develop in the areas around the shoulder blades and lower back, as these are the most common sites of vaccinations and other injections in our feline friends.

There is no definitive method to confirm a sarcoma’s relation to a previous vaccine, but the area in which it is found in the fibrous tissue is typically an indicator of the condition.

These masses are most commonly tied to routine vaccines in cats, but they can occur as a result of other injections as well.

How Are Vaccines Tied To Injection Site Sarcomas?

Unfortunately, research is still ongoing when finding the tie between vaccines and injection-site sarcomas in cats.

However, there are a few connections that have been established when discussing this rare reaction.

It seems as if some cats experience a prolonged inflammatory response after receiving certain killed vaccines.

Experts believe that these vaccines may stimulate an intense immune response, leading to the development of the injection-site sarcoma down the line.

Some experts tie the chronic inflammatory response to the aluminum that is present in some vaccines, as this helps to keep the vaccine in one spot in the tissue for a period after it is injected.

This could potentially lead to increased inflammation in the area, which is thought to be the cause of sarcoma development down the line.

Most recent studies have potentially linked injection site sarcomas to a tumor suppressive gene present in some cats, but evidence is not yet concrete.

This topic is still quite controversial in the veterinary world, as studies are still underway in attempting to understand this strange occurrence.

Are Injection Site Sarcomas Common In Cats?

When cat owners hear about the potential of injection-site sarcomas, some begin to question if vaccines are worth the risk.

Though injection-site sarcomas do occur in our feline friends, they are considered quite rare when looking at the numbers.

According to most reports about the prevalence of this condition, most believe that sarcomas occur in about 1 out of 10,000-20,000 vaccinated cats.

This number will vary based on the resource you reference, but this seems to be the average amount.

No matter which resource you turn to for research, each will agree that injection-site sarcomas are uncommon in our cat companions.

With these numbers in mind, it’s clear that the proven benefits of core vaccines far outweigh the risk of this reaction.

Common Signs Of Injection Site Sarcoma In Cats

The most obvious sign of an injection-site sarcoma in cats is a recently developed lump under the cat’s skin, but there are a few specific indicators to keep an eye out for as well.

To help you spot a growing sarcoma in your furry friend, let’s list a few of the most common symptoms below.

Signs of an injection-site sarcoma in cats include:

  • Lump under the cat’s skin
  • A fast growing lump under the skin
  • Irregularly shaped lump under the skin
  • Lack of pain when touching the lump (though it can become painful over time)

If your cat has an injection-site sarcoma that has spread to other parts of the body, you may notice some of the following symptoms as well:

  • Lethargy
  • Anorexia
  • Weight loss
  • Vomiting
  • Limping, especially when the lump is on or just above the leg
  • Respiratory changes

If you ever notice a new lump on our cat’s skins, we always suggest having it assessed by your veterinarian as soon as possible.

Even if the new growth is not a sarcoma, there are many other dangerous lumps and bumps to be concerned about.

How Long After Vaccines Do Injection Site Sarcomas Develop?

This can range from case to case, but it seems as if most injection-site sarcomas in cats will develop within 4 months to 4 years after they receive their vaccine.

Some sarcomas have been known to develop after the 5 year mark, but this is considered rare.

It’s also important to keep in mind that some cats will experience localized inflammation from their vaccine in the 3 month period after receiving it.

These immediate inflammatory responses will typically resolve on their own when they occur within the 2-3 month period after vaccination, but your vet will likely keep a close eye on the area to be safe.

If your cat has any inflammation in the area of their vaccine that does not resolve within 7-10 days of receiving it, we always suggest reaching out to your vet for guidance.

This can be normal for some cats, but it’s always best to be safe.

Diagnosing Injection Site Sarcomas In Cats

If your veterinarian has suspicions about a new lump on your cat’s skin, they will likely take a biopsy of the area and send it to a lab for microscopic testing.

A biopsy is the only way to definitely diagnose a fibrous sarcoma in our feline friends, and to rule out any other potential causes of the lump as well.

It’s important to note that while the sarcoma itself can be diagnosed, it is impossible to know whether or not it developed as a result of a previous vaccine or injection.

Your vet will often come to the conclusion of an injection-site sarcoma based on the location of the mass and your cat’s history with vaccines.

Your vet may also suggest x-rays to check for any evidence of metastasis, as well as routine blood work to rule out any other metabolic issues.

Treating Injection Site Sarcomas In Cats?

Treatment for your cat’s injection-site sarcoma will vary based on the location of the mass, the amount of time the mass has been present, and whether or not metastasis has occurred.

To help you better understand the treatment options available to your feline friend, let’s list the most common treatment tools below.

Treatment for injection-site sarcomas in cats may include a combination of the following treatment methods:

Surgical Mass Removal

Surgical removal of the sarcoma is typically the first treatment that is performed.

This involves not only removing the mass itself, but removing large margins to reduce the likelihood of recurrence.

Injection-site sarcomas are known for having deep rooted tumor cells in the surrounding areas, so deep and wide excision is essential.

Surgery alone is not often curative, and will require the use of radiation therapy or chemotherapy.


If the injection-site sarcoma is on the cat’s limb, amputation of the limb is often explored.

This helps to reduce the possibility of recurrence, and may even be necessary if the cancer has invaded the deep tissue.

This will vary from case to case, so it’s important to discuss all surgical options with your veterinarian.


If the sarcoma is labeled as aggressive based on biopsy results, your vet may suggest chemotherapy.

This typically includes anywhere from 4-6 rounds of treatment spaced 3 weeks apart, and is often well tolerated in our feline friends.


Radiation is the most effective way to target any cancerous cells that are left behind in the tissue after surgery.

Most treatment plans will involve radiation therapy each day for 3-4 weeks, with some days off in between to cater to scheduling and clinic office hours.

Life Expectancy Of Injection Site Sarcoma In Cats

Unfortunately, cats with injection-site sarcomas have a high likelihood of tumor recurrence.

Thorough treatment can keep cats tumor free for up to 24 months, but many will develop tumors again at some point down the line.

The use of radiation and chemotherapy with the first tumor growth is believed to keep cats tumor free for the longest period, which is why most vets will now use a combination of different methods to target the original tumor.

At the end of the day, experts state that the median survival time is more than two years.

Can You Reduce The Risk Of Injection Site Sarcomas In Cats?

Avoiding vaccines altogether is not ideal for most feline friends, leading many cat owners to wonder if there are any other ways to reduce the risk of injection-site sarcomas in cats.

Research is still being performed in regards to understanding the development of this condition, but there are some methods being practiced by vets today.

  • Only vaccinating cats with vaccines they absolutely need. This practice involves offering vaccinations against disease that your cat is most at risk to, and avoiding others that are not as much of a risk. For example, if you have an indoor cat that will never come in contact with other cats, some vets will suggest avoiding the FIV vaccine to decrease their vaccine load. 
  • Subcutaneous vaccines to allow for earlier detection. While subcutaneous vaccines versus intramuscular vaccines will not prevent sarcoma, these tumors may be detected earlier if they are closer to the top layer of the skin. 
  • Vaccinations below the stifle and closer to the limbs. This is still somewhat controversial in the veterinary world, as some vets do not believe this will make a difference. However, many state that amputation can reduce the risk of tumor recurrence when the mass is on the limb, so a vaccine closer to the limb may result in a more favorable outcome if a sarcoma develops down the line.

Final Thoughts

Injection-site sarcomas are an aggressive tumor that requires immediate attention.

Though research is still being performed, early detection is confirmed to offer your cat the best chance at recovery.

Every case will vary once it is diagnosed, so we always suggest following guidance of the vet that is caring for your feline companion.

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