Injection-Site Sarcoma (FISS) in Cats: What to Know

Quick Answer
  • Injection-site sarcoma (FISS) is a rare but aggressive soft-tissue cancer that can develop where a cat received a vaccine or other injection.
  • Most post-injection lumps are not cancer, but your vet should check any lump that lasts more than 3 months, grows after 1 month, or is larger than 2 cm.
  • Early planning matters. These tumors often send microscopic extensions into surrounding tissue, so the first surgery gives the best chance for control.
  • Treatment often involves staging tests plus wide surgery, and some cats also benefit from radiation therapy and sometimes chemotherapy.
  • Vaccines still protect far more cats than FISS harms. Prevention focuses on giving only needed vaccines, using risk-based schedules, and choosing injection sites carefully.
Estimated cost: $250–$800

What Is Injection-Site Sarcoma (FISS)?

Feline injection-site sarcoma, often shortened to FISS, is a malignant tumor that forms in tissues where a cat previously received an injection. It was first recognized around vaccine sites, so you may also hear vaccine-associated sarcoma. Today, vets use the broader term injection-site sarcoma because similar tumors have also been linked to other injections, including some long-acting medications.

These tumors usually arise from connective tissue cells such as fibroblasts and can behave aggressively where they start. That local behavior is what makes FISS challenging. Even when the lump on the surface looks small, the tumor may extend farther into nearby tissue and muscle than expected.

The good news is that FISS is rare. Reported risk estimates vary, but published veterinary sources describe it as occurring in roughly 1 to 16 cases per 10,000 vaccinations, with some client-facing sources citing about 1 in 1,000 to 1 in 10,000 vaccine injections. Even so, because early treatment improves planning and options, any persistent lump after an injection deserves attention from your vet.

Symptoms of Injection-Site Sarcoma (FISS)

  • Firm lump or swelling under the skin at a previous injection site
  • Mass that feels fixed to deeper tissue instead of freely movable
  • Lump that keeps getting larger over weeks
  • Mass larger than 2 cm (about the width of a grape to walnut, depending on shape)
  • Swelling that remains for more than 3 months after an injection
  • Pain, sensitivity, or discomfort when the area is touched
  • Reduced mobility if the mass is near a leg, hip, or shoulder
  • Skin ulceration or open, irritated tissue over the lump

Many cats with FISS seem normal except for a lump found during petting or grooming. A small bump right after vaccination can be a temporary inflammatory reaction, not cancer. The key question is whether it follows the 3-2-1 rule: see your vet if the lump is still there 3 months after the injection, is larger than 2 cm, or is still increasing in size 1 month after the injection. If the area becomes painful, ulcerated, or starts affecting movement, your cat should be seen sooner.

What Causes Injection-Site Sarcoma (FISS)?

FISS is thought to develop from a chronic inflammatory reaction in susceptible cats. The current understanding is not that one single vaccine or ingredient always causes cancer. Instead, some cats appear genetically or biologically predisposed, and prolonged inflammation after an injection may trigger abnormal transformation of fibroblasts and related cells.

Historically, FISS was most strongly associated with certain adjuvanted, inactivated rabies and feline leukemia (FeLV) vaccines, but experts now recognize that other injections can also be involved. Merck Veterinary Manual notes that long-acting corticosteroid injections and other injectable products have also been linked. At the same time, large studies have not proven that aluminum-containing adjuvants alone explain every case.

That is why prevention is about risk reduction, not eliminating all injections. Your vet balances the very small FISS risk against the very real danger of infectious diseases like rabies and FeLV. For many cats, vaccination remains clearly worthwhile, but the schedule, product choice, and injection location should be individualized.

How Is Injection-Site Sarcoma (FISS) Diagnosed?

Diagnosis starts with a careful history and physical exam. Your vet will want to know when the lump first appeared, whether it is growing, and what injections your cat received in that area. Because inflammatory nodules and sarcomas can feel similar early on, a hands-on exam alone is not enough to confirm the diagnosis.

Testing often begins with fine-needle aspiration, but this may not always give a clear answer because sarcomas can yield low-cell samples. If suspicion remains, your vet may recommend an incisional biopsy or tissue sample for histopathology, which is the test that confirms whether the mass is a sarcoma. Excisional removal of a small lump before planning can be a problem in FISS, because incomplete first surgery may reduce later local control.

Once FISS is confirmed or strongly suspected, staging helps guide treatment. This may include bloodwork, chest imaging to look for spread to the lungs, and often CT scanning or MRI to map how far the tumor extends locally before surgery. Referral to a veterinary surgeon or oncologist is often helpful because the first definitive treatment plan matters so much with this disease.

Treatment Options for Injection-Site Sarcoma (FISS)

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$800–$3,000
Best for: Cats whose families need a lower-cost path, cats with tumors in locations that are difficult to treat aggressively, or cats where the goal is comfort and time rather than maximal local control.
  • Exam and treatment planning with your vet
  • Needle sample or biopsy to confirm diagnosis
  • Basic staging such as chest X-rays and bloodwork
  • Palliative pain control and wound care if needed
  • Debulking or limited surgery only when referral-level treatment is not possible
Expected outcome: Comfort may improve, but local recurrence or continued tumor growth is common if margins are not wide. This approach is usually palliative rather than curative.
Consider: Lower upfront cost and less intensive care, but a higher chance the tumor will return quickly or remain incompletely controlled.
Most Common

Recommended Standard Treatment

$4,000–$8,500
Best for: Cats with localized disease where surgery can be planned early and performed by a surgeon comfortable with aggressive sarcoma removal.
  • Biopsy-confirmed diagnosis
  • Staging tests including chest imaging and lab work
  • Referral surgery with aggressive wide and deep margins
  • Pathology review of the removed tumor and margins
  • Follow-up monitoring for recurrence
Expected outcome: Best outcomes usually come from the first definitive surgery. Wide excision can provide meaningful control, especially when the tumor is found early and removed completely.
Consider: More invasive than palliative care and still not a guarantee. Even with surgery, recurrence remains possible because FISS often extends microscopically beyond the visible mass.

Advanced / Critical Care

$8,000–$15,000
Best for: Cats with potentially resectable tumors where pet parents want the fullest range of local-control options, or cases with recurrent disease or challenging tumor location.
  • Advanced imaging such as CT for surgical mapping
  • Radical surgery, sometimes including limb amputation or body-wall resection depending on location
  • Pre- or post-operative radiation therapy
  • Oncology consultation and discussion of chemotherapy in selected cases
  • Long-term recheck imaging and recurrence surveillance
Expected outcome: Combination therapy offers the longest tumor-free intervals for many cats. Published veterinary oncology sources report median survival often exceeding 1-2 years with surgery plus radiation, and some cats do better when the first treatment is aggressive and complete.
Consider: Highest cost range, more appointments, anesthesia events, and recovery demands. Chemotherapy benefit is less predictable than the benefit of well-planned surgery and radiation.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Injection-Site Sarcoma (FISS)

Bring these questions to your vet appointment to get the most out of your visit.

  1. Does this lump meet the 3-2-1 rule for biopsy or further testing?
  2. Would a fine-needle aspirate be useful here, or is an incisional biopsy the better next step?
  3. Should my cat have chest X-rays, CT, or other staging tests before surgery?
  4. Is referral to a surgeon or oncologist recommended before any mass removal?
  5. If surgery is planned, what margins are realistic for this location?
  6. Would radiation therapy likely improve local control in my cat's case?
  7. What is the expected cost range for conservative, standard, and advanced treatment options?
  8. How will this diagnosis affect future vaccine planning for my cat?

How to Prevent Injection-Site Sarcoma (FISS)

Prevention focuses on reducing unnecessary inflammation while still protecting your cat from serious disease. The most important step is a risk-based vaccine plan with your vet. Current feline vaccination guidelines support giving only the vaccines your cat truly needs and avoiding over-vaccination. For many adult cats, core vaccines are not given every year, and non-core vaccines like FeLV depend on lifestyle and exposure risk.

Injection location matters too. Feline vaccination guidelines recommend giving vaccines in sites that make future surgery more feasible if a sarcoma ever develops, rather than between the shoulder blades. Distal limb or tail locations may be used depending on the vaccine and the clinic's protocol. Your vet should also document what product was given and exactly where it was administered.

Pet parents can help by monitoring any post-injection lump at home. A small bump may fade on its own, but use the 3-2-1 rule and schedule a recheck if it persists, enlarges, or feels substantial. If your cat has had FISS before, future vaccine decisions should be especially individualized. In some cats, your vet may discuss non-adjuvanted vaccine options, spacing out needed injections, or avoiding nonessential injectable medications when reasonable.

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