CONDITION
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.
Why this matters now
Injection-site sarcomas tend to develop in adult and older cats, most often appearing between the ages of eight and twelve years, though younger cats can occasionally be affected. The time lag between injection and tumour detection varies considerably; some masses are noticed within a few months, whilst others may not become apparent for several years. There is no strong breed predisposition, and both neutered and intact cats appear equally susceptible. The key trigger is thought to be the local tissue inflammation that follows certain injections, particularly adjuvanted vaccines or long-acting injectable medications.
These tumours often grow slowly at first, which can make early lumps easy to overlook or attribute to a simple reaction. Once established, however, they tend to enlarge steadily over weeks to months, and their growth pattern is characteristically invasive—extending along tissue planes and sending microscopic projections beyond the palpable mass. Because of this infiltrative behaviour, complete surgical removal can be difficult, and recurrence at the same site is not uncommon even after initial treatment. The biological behaviour varies between individuals; some tumours remain localised for extended periods, whilst others grow more aggressively or, in a small proportion of cases, spread to distant sites such as the lungs or lymph nodes.
Signals & patterns
Early signals
Firm lump under the skin
The owner may notice a small, firm swelling at a site where the cat has previously received an injection—commonly between the shoulder blades, along the flank, or on a hind leg. The lump often feels distinct from the surrounding tissue and does not seem painful when touched.
Lump persisting beyond healing time
A raised area or swelling that remains unchanged or continues to be palpable more than three months after an injection may warrant attention, as benign post-injection reactions typically resolve within a few weeks. Persistence alone does not confirm a sarcoma, but it marks a shift in the expected timeline.
Gradual increase in size
An initially small lump that grows steadily over the course of weeks or months, rather than shrinking or stabilising, can signal a process beyond normal healing. The rate of growth varies, and some masses enlarge very slowly in the early stages.
Lump becoming more obvious
As the mass grows, it may become easier to see as well as feel, creating a visible bulge beneath the fur or causing asymmetry in the body contour. This change often prompts closer observation by the owner.
Later signals
Rapid enlargement of the mass
A lump that has been stable or slow-growing may begin to increase in size more quickly, doubling over the course of weeks rather than months. This shift in tempo can reflect progression within the tumour itself.
Attachment to underlying tissue
The mass may feel less mobile and more firmly anchored to deeper structures such as muscle or fascia, making it difficult to shift beneath the skin. This reflects the tumour's tendency to infiltrate surrounding tissue planes.
Ulceration or skin changes
In some cases, the overlying skin may become stretched, thinned, or broken, leading to visible sores or discharge. This typically occurs when a tumour has grown large enough to compromise the integrity of the skin above it.
Discomfort or lameness
If the mass is located near a limb or involves muscle or nerve tissue, the cat may begin to show reluctance to move, stiffness, or favouring of the affected leg. Pain is not always present but may develop as the tumour expands or invades sensitive structures.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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