Injection-Site Sarcoma in Cats: The One You've Never Heard Of

A feline injection-site sarcoma, often shortened to FISS, is a cancer that arises from the connective tissue under the skin, at a spot where an injection has been given. Most are a type called a fibrosarcoma (NC State University College of Veterinary Medicine). It seems to start as a reaction to inflammation at the injection site that, very occasionally, tips over into a tumour over months to years.

The thing that makes it matter, despite being rare, is its character. FISS is locally aggressive, meaning it tends to dig roots out into the surrounding tissue rather than sitting as a tidy ball, which is what makes it stubborn to remove fully. It does sometimes spread to other parts of the body, in up to around a quarter of cats, but its main trick is coming back at the same spot if it isn't taken out widely enough the first time (NC State University College of Veterinary Medicine). That local stubbornness is the whole reason it's treated so seriously.

And although vaccines are the injections most often discussed, FISS has been linked to other injected products too, including some long-acting medications. The common thread is the injection and the inflammation it causes, not vaccines specifically.

Rare. The real difficulty is that the published estimates sit across quite a wide range, because it's so uncommon that it's hard to count precisely. A good UK study put the risk at somewhere between 1 in 5,000 and 1 in 12,500 vaccination visits (Dean, Pfeiffer & Adams, 2013). The American Veterinary Medical Association puts it at roughly 1 cat in every 10,000 to 30,000 vaccinations (AVMA). Either way, the overwhelming majority of cats vaccinated every year never develop one.

It's also a risk that has been getting smaller. The AVMA notes that improvements in vaccines and in how they're given have greatly reduced it, including newer vaccine technology (so-called recombinant, non-adjuvanted vaccines, which leave out the irritant "adjuvant" that older killed vaccines used to boost the response) and more thoughtful schedules, where your vet gives only the vaccines your individual cat actually needs (AVMA).

Here's the genuinely useful part. A small, soft, slightly tender swelling at an injection site in the first days or weeks afterwards is usually just the body's normal reaction, and it settles on its own. You don't need to panic about it. What you do is watch it, and apply a simple rule that vets use, often called the 3-2-1 rule.

The 3-2-1 rule says to have a lump at an injection site checked, and usually sampled or biopsied, if any one of these is true (Today's Veterinary Practice, 2021):

• it's still there 3 months or more after the injection,
• it's bigger than 2 centimetres across, or
• it's still growing 1 month after the injection.

Tick any one of those boxes and it's worth a proper look, which usually means a sample to find out what the lump is made of (our piece on aspirates and biopsies explains what that involves). Most lumps that get checked turn out to be harmless. But because catching a FISS early and small makes it so much easier to treat, this is the rare situation where "wait and see" really isn't the plan. A quick way to stay on top of it is to note where each injection went and to measure and photograph any lump in the Lump & Bump Tracker, so you and your vet can see at a glance whether it's changing.

If a sample does confirm a sarcoma, the single most important thing to understand is that this is not a lump you simply pop out. Because FISS sends those finger-like roots into the surrounding tissue, a small "shell it out" operation almost always leaves cancer cells behind, and it grows straight back.

The treatment that actually works is wide, sometimes radical, surgery: the surgeon removes a generous margin of normal-looking tissue all around and beneath the tumour, often around 5cm in every direction and a layer or two of muscle deep (NC State University College of Veterinary Medicine; Today's Veterinary Practice, 2021). When that's done thoroughly, the great majority of tumours are fully removed, with one approach achieving complete margins in around 97% of cases, though even then about 14% still come back, which is why follow-up matters (Today's Veterinary Practice, 2021). It's often combined with radiotherapy before or after the operation to mop up stray cells, and sometimes chemotherapy too, particularly for higher-risk tumours (NC State University College of Veterinary Medicine). Handled by a specialist team this way, many cats do well: surgery alone gives an overall survival of around 12 to 16 months, and adding chemotherapy and radiation can stretch the tumour-free interval to roughly 18 to 24 months (NC State University College of Veterinary Medicine). The headline is that the first surgery is the best chance, so a confirmed FISS belongs with a surgeon or oncologist who treats them, not a wait-and-see plan (our article on cancer surgery and clean margins explains why that first attempt counts for so much).

This is the part to hold on to, because it's the whole point of telling you any of this calmly rather than scaring you off.

The diseases that core vaccines prevent (things like feline panleukopenia, cat flu and rabies where it's relevant) are common, they spread easily, and they kill cats. The risk of a sarcoma is, by contrast, very small. As one university cancer centre puts it, the risk of not vaccinating against certain diseases may be far higher than the risk of a tumour developing (NC State University College of Veterinary Medicine). Leading veterinary bodies say the same: vaccination remains one of the most important parts of keeping a cat healthy, and you should keep vaccinating (AVMA). Abandoning your cat's vaccines to avoid a rare cancer would be swapping a small risk for some genuinely large ones.

What's reasonable, and worth a relaxed chat with your vet, is making sure your cat gets the vaccines she actually needs for her lifestyle and no extras, and asking whether the newer non-adjuvanted vaccines are an option. You may also notice your vet now gives injections low down on a leg, or even into the tail, rather than between the shoulder blades. That isn't because something's wrong, it's a deliberate, sensible change, so that in the very unlikely event a sarcoma ever did form, it would be in a spot where it could be removed completely (AAHA/AAFP, 2020; Today's Veterinary Practice, 2021).

Keep it simple. Carry on protecting your cat with the core vaccines she needs, and treat a fresh post-jab lump as a watch-and-check, not a crisis. Make a mental note of roughly where each injection goes, run any lump past the 3-2-1 rule, and if it ticks even one box, ask your vet to take a sample rather than leaving it. You can log and photograph it in the Lump & Bump Tracker in the meantime so the picture's clear. The reassuring truth is that almost every lump you'll ever feel after an injection is harmless, and the rare one that isn't is most beatable exactly when it's caught the way this page describes: early, and small.