Sarcoma is a group of malignant neoplasms of mesenchymal origin — affecting tissues like muscle, fat, vessels, nerves and connective tissue. In cats, they represent an important chapter of oncology, especially because of feline injection-site sarcoma (FISS), a clinical entity that has changed feline vaccination protocols worldwide.
This guide reviews the main types of sarcoma in felines, with emphasis on FISS, and organizes what matters in practice: how to suspect, how to confirm and how to guide the owner.
Main types of sarcoma in cats
Soft tissue sarcoma (STS)
Umbrella term for mesenchymal neoplasms including fibrosarcoma, hemangiopericytoma, myxosarcoma, liposarcoma and others. They usually present as a firm, infiltrative, progressively growing mass with ill-defined borders. Location varies: trunk, limbs, head.
Fibrosarcoma
The most common subtype in cats. Can be cutaneous, subcutaneous or oral. Oral fibrosarcoma deserves special attention: it often occurs in older cats, on the gingiva or mucosa, with locally aggressive behavior and high recurrence rate even after surgery.
Osteosarcoma
Less common in cats than in dogs, but exists. Usually involves bones of the axial and appendicular skeleton. In felines, prognosis tends to be better than in dogs after amputation alone, with significantly longer survival — but early diagnosis remains decisive.
Feline Injection-Site Sarcoma (FISS)
A clinical entity unique to cats. An aggressive, infiltrative tumor arising at sites where vaccines (especially adjuvanted, such as FeLV and rabies) or other injections (long-acting corticosteroids, for example) were administered.
Clinical signs of FISS — the "3-2-1" rule:
Consider biopsy if a mass at an injection site:
- Persists for more than 3 months after the injection
- Is larger than 2 cm in diameter
- Increases in size 1 month after appearing
This rule is the international standard adopted by feline oncology societies for early diagnosis.
Diagnosis
Basic steps
- Detailed history: record exact location and date of prior injections (vaccines, long-acting injectables)
- Physical exam: measure the mass, palpate borders, evaluate mobility and adherence to deep planes
- Fine needle aspirate (FNA): useful as initial screening, but often inconclusive in STS — yielded material tends to be sparse
- Incisional biopsy: gold standard. Always planned to avoid compromising definitive surgery (incision line within the future surgical field)
- Histopathology with grading: defines subtype and grade, which directly impact prognosis
- Staging: 3-view thoracic radiographs and, ideally, regional CT for surgical planning in FISS
Treatment — general principles
Surgery
- Wide and deep margins (in FISS, recommended 3–5 cm lateral and 2 fascial planes of depth)
- Histopathology with margin assessment is mandatory — compromised margins change the treatment plan
- Recurrences are usually worse than the primary tumor; the first surgery being well done is worth it
Radiotherapy
Adjuvant recommended when margins are compromised, tumors are high grade or in anatomically difficult locations for wide resection.
Chemotherapy
Considered in high-grade tumors, recurrences or metastatic disease (rare, but exists — especially in FISS).
FISS prevention — practical protocol
WSAVA and AAFP recommend:
- Apply vaccines in distal sites (as distal as possible on limbs or tail) that allow amputation if FISS develops
- Document which vaccine was given at which site in every vaccination record
- Avoid interscapular vaccination (between scapulae) — a classic FISS site with much more difficult surgery
- Prefer non-adjuvanted vaccines when available
- Do not apply multiple vaccines at the same site
Owner guidance
The owner needs to understand three points:
- Any "lump" after vaccination deserves attention if it persists, grows or exceeds 2 cm — return to clinic for reassessment
- Early surgery with adequate margins is the best predictor of cure
- Post-surgical follow-up is lifelong — recurrences can occur months after resection
The role of structured clinical records
Feline oncology cases demand impeccable record-keeping: date and location of every injectable application, mass dimensions at each follow-up, photographic evolution, histopathology reports and margins. The more organized the record, the sooner patterns emerge — and the sooner the 3-2-1 rule is applied.
In feline sarcomas, especially FISS, the difference between cure and recurrence often lies in the consistency of follow-up. A well-kept medical record is part of the treatment.
