Cat Injection Site Sarcoma: the Risks, Treatment + Prognosis of this Deadly Tumor

An injection site sarcoma is a nasty, aggressive tumor. You may have heard that vaccines are responsible for their formation. Is this true, and what can we do to reduce the risk of your cat developing this aggressive mass?

injection site sarcoma in cats

My understanding was that the majority of injection-related sarcomas were linked to the use of adjuvants in vaccines (which unfortunately are still used in some veterinary practices, despite the existence of safer, non-adjuvanted alternatives). - Ken

This question comes after another one of my questions about the side effects of vaccination, where I did talk about injection sites sarcomas rather than vaccine-related sarcomas.

Cat Vaccine or Injection Site Sarcoma

Cats get this phenomenon called injection site tumor, injection site sarcomas or injection site fibrosarcoma. You might also see it written as FISS, standing for feline injection-site sarcoma.

These tumors develop in about one in 10,000 to 30,000 vaccinated cats. That’s 0.00003 - 0.0001%! They are very rare, but there is a real risk.

It was first thought that these tumors were completely vaccine-related. That vaccines were responsible for causing these tumors in these unfortunate cats. It has now become much clearer that it is actually simply the act of having any form of an injection that can trigger this cancer formation.

Rather than even being injected with anything, it may be just the act of the needle going in, or it may be the injection of any substance that is triggering this cancer formation.

It is not just vaccines.

Vaccines are just the most common injection that most cats will get. Certainly, younger cats are really not going to be getting many other injections at all. The vaccines may make up a large percentage of vaccines while they are young.

This is one reason why vaccines were the first to be implicated

Injection site sarcomas have also been found to develop after a range of other drug injections. They've been found around suture material that has been used in surgery. There is even one case of injection sites sarcoma following microchip implantation.

The bottom line is that a wide range of injections are known to result in this disease.

What about Adjuvants?

As Ken suggested, what about the role that adjuvants might play? These have been strongly implicated in the past.

This is actually really unclear, and this includes aluminum adjuvants or any other adjuvants. If you’re not aware what an adjuvant is, they are substances that are administered as part of a vaccine to enhance it’s effect, stimulating a better immune response with more antibodies being produced along with a longer lasting immunity. Adjuvants are designed to simply make a vaccine more effective.

It is not clear that any adjuvants are responsible to any greater degree than some of the other substances injected that can cause an injection site sarcoma in cats.

Having said that, it does appear that these tumors are more likely following leukemia and rabies vaccinations. Both of which contain adjuvants.

Causes and Symptoms

The cause of injection-site sarcoma in cats is unclear. It is thought that the inflammation that can result following the injection of any substance, or the act of having a needle and act of being injected, is actually just not controlled properly.

Is there an actual problem within a certain line of cats? Are certain cats genetically predisposed to developing these tumors, or genetically more likely to suffer from injection site sarcomas compared to other cats?

We just don’t know.

Whatever’s happening, this inflammation may be resulting in cell mutation and then cancer formation.

Signs of Sarcoma

A small lump immediately following vaccination or any injection is not unusual, but it should not persist for any longer than a couple of weeks.

3-2-1 Rule

We can think about something called the 3-2-1 rule. This rule gives us a guideline as to when to investigate any potential injection site sarcoma.

  • If a lump is still present three months after injection

  • If the size of the lump is larger than two centimeters at any time point. If it's grown to two centimeters very quickly, or if it is growing slowly and becomes larger that 2cm then investigation is needed

  • If a mass is increasing in size one month following injection

All of these are triggers that should prompt an investigation

“Normal” reactions

In most cases, if we got a swelling after injection or after vaccination, it will be smaller. It will not persist for that length of time and it is not going to be increasing in size a month after injection. 

Those are all things that are not going to happen in a normal case. If those are happening, then we absolutely need to get onto it straight away at the earliest opportunity.


The reason rapid diagnosis is important is that these cat injection site sarcomas are aggressive tumors that need prompt, aggressive surgery.

Ideally, surgery will be guided by CT or MRI scanning so that your vet can see the exact nature of the tumor. They can try and work out exactly where it is so that it can be removed completely during surgery.

The reason this can be a challenge is that sometimes there are little tendrils or little tentacles sent out in different directions, which are very difficult if not impossible to pick up either before surgery or even during surgery by site and feel alone. If we can get some advanced imaging, then we're going to have a much better chance of removing it completely by visualizing exactly where the tumor is invading.

Aggressive surgery can result in a post surgery tumor-free period of over 300 days, compared to only about 80 days if only marginal surgery is able to be performed.

Either way, surgery needs to be aggressive if at all possible (which isn’t always the case) to stand the best chance of successfully removing the tumor in its entirety. Be prepared for a big scar.

Post Surgery Treatment

After surgery, there is the potential for other follow-up treatments such as chemotherapy or radiation therapy. That is going to depend on the surgery, how that went and what resources are available in your area. Costs are also clearly going to be a consideration.

Radiation may be able to improve post surgery life expectancy significantly. Raising it to around 2 years, compared to only 1 year without radiation (source). Evidence though is mixed, with original tumor size potentially playing the most important role in survival, rather than the post-surgery treatment given.

The bottom line is that these are invasive, nasty tumors. They are aggressive and there is a high risk of recurrence. Because of those little tentacles, it is very difficult to get every single tumor cell, which means that there is a reasonable chance of the tumor coming back.

That highlights why we want to be getting onto this at the earliest opportunity just to give ourselves the best chance of removing it completely.

And then finally, 25% of tumors (1-in-4) will actually metastasize. This means that they will spread to other parts of the body. Once that happens, it’s bad news.

Reducing Tumor Risk

As a result of feline injection site sarcomas, there has been a move to change where we are actually injecting cats. It's moved from the scruff, the traditional injection site between the shoulder blades where there is a lot of skin that are not very sensitive.

Instead, in most cases we are trying to move to the lower thigh or to the lower front leg if possible. The reason being that is an injection site sarcoma does develop because that is going to make any tumor easier to completely remove through amputation. While that might seem really extreme, it's very difficult to remove this tumor in any other location.

Cats will do very, very well with three legs in the vast majority of cases.

The difference in tumor location is a stark as a cat who survives with 3 legs, compared to a cat who dies as a result of the injection site sarcoma. Improving survival can only be a good thing in this case.

The Bottom Line

Injection site sarcoma in cats is a serious, aggressive, frequently fatal cat tumor.

But remember, this is a very rare tumor affecting less than 0.0001% of cats and we can't lose sight of the fact that we're giving any injection.

A lot of the diseases that we are vaccinating cats against are life-threatening, life-limiting, or have a massive impact on the quality of life, as well as being are very common. In the case of other injections, they are clearly needed if your cat is unwell. Sometimes giving an injection is the only option to effectively treat a cat.

We do not want to have the extreme reaction of saying vaccines (or any injection) are completely unnecessary because we don't want to risk our cat getting a tumor. The risk of disease is going to be higher than the risk of a tumor developing.

That being said, we also need to give our vaccines at an appropriate interval. We do not want to give more vaccines than our cat needs based on their lifestyle, and we don't want to give them more frequently than necessary.

The above is a transcript taken from “The Dr Alex Answers Show”.

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