Injection-site sarcoma (also called feline injection-site sarcoma or FISS) is a cancer that develops in the tissue where a cat previously received an injection — most commonly a vaccine, but also potentially long-acting steroid injections or microchip implants. The tumour is believed to arise from an abnormal inflammatory and fibrotic response to the injected material in genetically susceptible cats. It typically appears as a firm, progressively enlarging mass at a common injection site, most often between the shoulder blades, though current vaccination guidelines now recommend administering vaccines in limbs so that wider surgical excision is possible should this rare complication develop. The incidence is low (1–4 per 10,000 vaccinated cats), but the tumour is locally very aggressive, with microscopic extensions reaching far beyond the palpable mass. The first surgery offers the best chance of cure, and wide surgical margins (5 cm in all directions) are essential.