Feline injection-site sarcoma

What it is

Feline injection-site sarcoma (FISS) is a malignant mesenchymal tumour that arises at an injection site, mainly of adjuvanted vaccines (historically inactivated rabies and feline leukaemia vaccines), but also of other products such as long-acting antibiotics or corticosteroids. Chronic local inflammation in certain predisposed cats is thought to drive a proliferation of fibroblasts and myofibroblasts that eventually becomes malignant. Histologically it is usually a fibrosarcoma, although other sarcomas are seen; it is more aggressive than feline sarcomas unrelated to injections.

The tumour grows in a highly infiltrative way: it sends microscopic projections along fascial planes far beyond the palpable mass. This is why surgery with narrow margins almost always leaves tumour behind and recurrence is the rule. Metastases (lung, lymph nodes) occur in a proportion of cases, usually late.

Who it affects

Adult cats, at a variable interval after the injection (months to years). It is uncommon relative to the number of vaccines given, but the risk justifies prevention measures. There is no clear breed or sex predisposition.

Anatomy involved

• Skin and subcutis: a firm subcutaneous mass, attached to deeper planes.
• Interscapular region: the classic site, because vaccines used to be given there; the tumour invades the thoracolumbar fascia, the epaxial muscles, the latissimus dorsi and the spinous processes of the thoracic vertebrae and the scapula.
• Thoracic and abdominal wall, limbs: other injection sites.

Clinical signs

A firm, painless, subcutaneous or intramuscular mass at an injection site that grows quickly. It may ulcerate or have cystic areas. Systemic signs (weight loss, dyspnoea) appear only in advanced stages or with metastasis.

Diagnosis

• 3-2-1 rule: biopsy any mass at an injection site that persists more than 3 months after injection, is larger than 2 cm or is still growing 1 month after injection.
• Incisional biopsy (core or wedge) before surgery, placed so that the tract can be removed later. Excising the mass without a prior diagnosis is not recommended, because it compromises curative surgery.
• Contrast-enhanced computed tomography to assess the true extent and plan surgery or radiotherapy; thoracic radiographs or CT to look for metastases.

Treatment

The first surgery has the best chance of cure. Radical excision is recommended, with lateral margins of about 5 cm and two fascial planes deep, which often includes resecting muscle, spinous processes, part of the scapula or of the thoracic or abdominal wall; on a limb, amputation. Marginal excisions have very high recurrence rates. Radiotherapy before or after surgery improves local control, and chemotherapy (doxorubicin, among others) is used as an adjunct or for palliation, with limited benefit. The prognosis improves greatly with early radical surgery; recurrences tend to be more aggressive.

Prevention: give only the vaccines the cat needs according to its risk, at appropriate intervals; choose non-adjuvanted vaccines where available; inject in the distal limbs (below the stifle or elbow) or the tail, never in the interscapular region, so that a tumour could be removed completely; and record the site of every injection.

Key studies

Frequency, risk and prevention

• Emdin et al. (2026), a systematic review and meta-analysis of 8 studies covering more than 14 million vaccinations and 411 sarcomas. The pooled incidence was 8.8 cases per 100,000 vaccinations (95% CI 0.8–100.6), about 1 in 10,000. Differences between studies were very large and the certainty of the evidence was very low.
• ABCD guidelines (Hartmann et al., 2015): they consider chronic inflammation the trigger and put the metastasis rate at 10–28%. The risk is higher with adjuvanted vaccines and lower with modified-live and recombinant vaccines, although no vaccine is risk-free. They recommend vaccinating as often as necessary but as rarely as possible, preferring non-adjuvanted vaccines, injecting where surgery could be curative and avoiding the interscapular region.
• Srivastav et al. (2012), a case-control study of 181 cats with sarcoma and two control groups. In the interscapular region, cases had more often received long-acting corticosteroids. In the pelvic limbs, cases were much less likely to have received recombinant rather than inactivated rabies vaccine (OR 0.1). No vaccine was risk-free.
• Shaw et al. (2009), 392 sarcomas at potential injection sites, before and after the 1996 recommendations of the North American Vaccine-Associated Feline Sarcoma Task Force. The interscapular proportion fell from 53.4% to 39.5% and tumours on the limbs increased, but so did tumours on the lateral abdominal wall, which are harder to remove, probably from injections aimed at the pelvic limb but given too high.

Surgery

• Phelps et al. (2011), 91 cats treated with radical excision with 5 cm margins alone. 14% had local recurrence and 20% metastasis; median survival was 901 days (1461 without recurrence and 499 with it). 10 cats had major complications, mostly wound dehiscence.
• Müller and Kessler (2018), 131 cats operated with lateral margins of at least 3 cm and two fascial planes deep (or full-thickness body wall). 38.1% recurred: 30.5% when tumour-bed biopsies contained no tumour and 76.2% when they did. Operating on a recurrence carried a worse prognosis than operating on a new tumour (further recurrence in 55.5% versus 33.3%). The two studies differ in margins and population, so their rates are not directly comparable.