Injection-Site Sarcoma in Cats

An injection-site sarcoma is a type of tumour that can develop in cats at locations where injections have been given in the past. These growths arise from the soft tissue beneath the skin and tend to grow locally, often with irregular borders that make complete removal challenging. The link between certain vaccines or injectable medications and these tumours was recognised in the 1990s, though they remain uncommon overall. Owners typically notice a firm lump or swelling under the skin, often between the shoulder blades, along the back, or in the hind leg—areas where vaccines and other injections are commonly given. The lump may appear weeks, months, or even years after an injection. Not every lump at an injection site is a sarcoma; many are benign reactions that resolve on their own, but lumps that persist, grow, or appear after the expected healing window tend to prompt further investigation. This page explores what changes may signal concern, what is understood about how these tumours develop, how they are investigated through imaging and biopsy, and the range of approaches available—including surgery, radiation, and chemotherapy—each with its own scope and tradeoffs.