Injection-site sarcoma in cats
Also known as FISS, Post-vaccination fibrosarcoma.
An aggressive and uncommon tumour (in the region of one to ten per ten thousand vaccinated cats) that can appear where an injection was given. That is why vaccines are now given in the legs or tail.
- System
- Tumours
- Is it urgent?
- Not usually
- Spreads to other cats
- No
- Spreads to people
- No
- Typical cost in Spain
- Biopsy, €100–250; wide surgery, €1,000–3,000; radiotherapy, more.
Symptoms
- A lump at an injection site that does not go away
- A lump that grows or is larger than 2 cm
- Firm and painless
Causes
Chronic local inflammation that, in predisposed cats, ends up becoming a tumour. It has been described with vaccines and with other injectable products.
How it is diagnosed
The 3-2-1 rule: biopsy if the lump lasts more than 3 months, is larger than 2 cm or is growing one month after the injection.
Treatment
Surgery with very wide margins, often with radiotherapy.
No doses, on purpose: any medicine must be prescribed by your vet for your cat.
Prognosis
Guarded: it tends to come back if it is not removed with clear margins.
Prevention
Vaccinate only as needed for the cat’s risk, in the legs or tail, and keep an eye on post-vaccination lumps.
Frequently asked questions
What causes injection-site sarcoma?
Chronic local inflammation that, in predisposed cats, ends up becoming a tumour. It has been described with vaccines and with other injectable products.
Can injection-site sarcoma be cured?
Guarded: it tends to come back if it is not removed with clear margins. Surgery with very wide margins, often with radiotherapy.
How much does it cost to treat injection-site sarcoma?
As a guide for Spain in 2026: Biopsy, €100–250; wide surgery, €1,000–3,000; radiotherapy, more. Prices vary a lot between clinics and cities, and out-of-hours emergencies cost more.
How can injection-site sarcoma be prevented?
Vaccinate only as needed for the cat’s risk, in the legs or tail, and keep an eye on post-vaccination lumps.
More in tumours
Educational content from Michipedia. It does not replace diagnosis or treatment by a vet: if you have any doubt about your cat's health, ask. Costs are a guide for Spain in 2026.