Serious severity
Feline Injection-Site Sarcoma
A rare but aggressive tumour that can develop at the site of an injection, months or even years later. Early surgery gives by far the best chance of a cure.
⚠️ Common symptoms
Small, temporary lumps after an injection are common and usually settle within a few weeks. Very occasionally, one does not — and feline injection-site sarcoma is the reason vets take persistent lumps seriously.
These are soft-tissue tumours that arise in connective tissue at a site of chronic inflammation. The prevailing theory is that in a genetically susceptible minority of cats, a prolonged local inflammatory response triggers malignant transformation. Adjuvanted vaccines were the first association recognised in the 1990s, but long-acting injectable drugs and other sources of persistent local irritation have since been implicated too, which is why the condition was renamed from 'vaccine-associated' to 'injection-site' sarcoma.
Their behaviour is characteristic: they invade locally and extensively, sending fingers of tumour well beyond what can be felt, but they metastasise relatively infrequently. That combination has a clear clinical consequence — the goal is a single, radical, well-planned surgery that removes the tumour with a generous margin of normal tissue in every direction. A conservative 'lumpectomy' almost always leaves cells behind, and the recurrence that follows is harder to treat than the original. This is why referral to a specialist surgeon, often with radiotherapy alongside, is the standard recommendation.
For owners, two things matter. The first is the 3-2-1 rule: check any injection-site lump that is still present at three months, larger than 2 cm, or growing at one month. Early, small tumours have the best outcomes by a wide margin. The second is perspective. This tumour is rare, and vaccination protects against panleukopenia, cat flu and other diseases that kill far more cats than sarcoma ever will. The modern answer is not to avoid vaccinating but to vaccinate thoughtfully — appropriate frequency, appropriate product, and a limb site where a cure remains possible.
🔍 Causes
The tumour is thought to arise from chronic inflammation at an injection site, which in a small number of genetically susceptible cats triggers malignant change in connective tissue. Adjuvanted vaccines and long-acting injectable medications have both been implicated, as have other causes of persistent local inflammation such as microchips or retained suture material. It is genuinely rare — estimates range from roughly one in a thousand to one in ten thousand vaccinations.
🛡 Prevention
The risk cannot be eliminated, but it is managed. Modern practice is to vaccinate only as often as a cat's lifestyle requires, to prefer non-adjuvanted vaccines where available, and to give injections in a distal limb or the tail rather than between the shoulder blades, so that amputation remains a curative option if a tumour ever develops. Vets are also encouraged to record exactly where each injection was given. None of this is a reason to skip core vaccination — the diseases prevented are far more likely to harm a cat than this rare tumour.
💊 Treatment
Treatment is aggressive surgical removal with wide margins of healthy tissue around and beneath the tumour, ideally performed by a specialist surgeon at the first attempt — repeat surgeries after an incomplete first excision have markedly poorer outcomes. Radiotherapy before or after surgery is commonly used to reduce the chance of regrowth, and chemotherapy may be added in higher-risk cases. These tumours regrow locally far more often than they spread, so getting the first operation right is the single most important factor.
⚕️ Medical disclaimer
- This information is educational only and is not a substitute for veterinary advice.
- Symptoms overlap between conditions — only a vet can diagnose accurately.
- Never withhold treatment or self-medicate based on information found online.
- When in doubt, call your vet. Early intervention almost always leads to better outcomes.