Feline injection-site sarcoma
Also known as: Feline Injection-Site Sarcoma, FISS, Vaccine-Associated Sarcoma, VAS, Fibrosarcoma
In short
Feline injection-site sarcoma (FISS) is a rare but highly aggressive cancer that can develop at injection sites in cats. Learn how to spot the signs using the "3-2-1 rule" and understand the diagnostic and treatment options available.
TL;DR. Feline injection-site sarcoma is a rare but highly aggressive cancer that develops at previous injection sites in cats, requiring prompt detection via the "3-2-1 rule" and aggressive veterinary treatment.
Injections are typically administered into the subcutaneous tissue or muscle layers of a cat.
Feline injection-site sarcoma (FISS)—also historically referred to as vaccine-associated sarcoma (VAS) or feline fibrosarcoma—is a highly aggressive, locally invasive malignant tumor. This cancer originates in the subcutaneous connective tissue (the layer of tissue just beneath the skin) or within the muscle layers of cats. It specifically develops at sites where the cat has previously received an injection or vaccination.
To understand this disease, it is helpful to look at how the feline body responds to injections. When a cat receives a vaccine or medication, the body naturally mounts a localized inflammatory response. In a very small percentage of cats, this inflammatory process does not resolve normally. Instead, the chronic inflammation triggers a genetic mutation in the surrounding connective tissue cells (fibroblasts), causing them to divide uncontrollably and form a malignant tumor.
These tumors are characterized by their "locally invasive" nature. While some cancers form neat, self-contained lumps, injection-site sarcomas send microscopic, finger-like projections deep into the surrounding tissues, muscles, and even bone. This makes complete surgical removal exceptionally challenging, as tumor cells often remain behind even after what appears to be a successful surgery. For cat owners, understanding this disease is vital because early detection is the single most critical factor in determining a cat's long-term survival.
The primary trigger for feline injection-site sarcoma is the administration of an injection. While vaccines (particularly those for rabies and feline leukemia virus) are the most commonly discussed triggers, any injection—including long-acting antibiotics, corticosteroids, or even microchips—can potentially initiate the inflammatory cascade that leads to tumor formation.
Despite this link, it is crucial to recognize that FISS is a rare condition. A leading veterinary internal medicine reference notes the estimated incidence rate:
"In this syndrome fibrosarcomas (FSAs) or other types of sarcomas develop in the subcutis or muscle in the interscapular region or the thigh, common sites of injection/vaccination. It is estimated that a sarcoma develops in 1 to 2 of 10,000 cats that receive an injection. Although the exact pathogenesis is still unclear, both the adjuvants and the local immune response against the antigens (i. e. ,"
Adjuvants are substances added to vaccines to stimulate a stronger immune response, which helps provide longer-lasting protection against infectious diseases. However, in susceptible cats, this heightened local immune response and the resulting chronic inflammation are believed to drive the transformation of normal cells into cancerous ones.
There are currently no documented breed predispositions for feline injection-site sarcoma. Any cat receiving an injection is theoretically at risk, regardless of breed, age, or sex. Because vaccinations are essential for protecting cats from deadly infectious diseases, veterinary organizations do not recommend avoiding vaccines. Instead, they focus on safe administration protocols and vigilant monitoring.
The clinical signs of feline injection-site sarcoma are primarily physical changes at the site of a previous injection. Because these tumors grow rapidly, owners must inspect their cats regularly, especially in the weeks and months following veterinary visits.
A firm, rapidly growing lump at a previous injection site is the primary clinical sign of feline injection-site sarcoma.
If you detect a lump on your cat, your vet will perform a thorough physical examination and recommend diagnostic testing. Because feline injection-site sarcomas do not behave like typical benign lumps, a precise diagnostic pathway is essential.
The diagnostic process often begins with a fine-needle aspirate (FNA). During this quick procedure, your vet inserts a thin needle into the mass to collect cells for microscopic evaluation. However, FNAs are not always definitive for sarcomas. As a leading veterinary internal medicine reference explains:
"The current recommendation is to use the "3, 2, 1 Rule": worry if the mass persists for more than 3 months after vaccination, is larger than 2 cm in diameter, or grows 1 month after the injection. Although FNA findings may provide a definitive answer, more often a surgical biopsy is necessary because sarcomas do not consistently exfoliate cells (see Chapter 71). Although most FSAs in dogs and cats"
Because sarcoma cells are tightly bound within a fibrous matrix, they do not easily detach (exfoliate) into a needle. Therefore, the gold standard for diagnosing FISS is an incisional biopsy or a surgical biopsy. An incisional biopsy involves surgically removing a small wedge of the tumor while leaving the rest intact. This tissue sample is sent to a veterinary pathologist, who can confirm the diagnosis of a sarcoma and determine its specific grade.
Once a diagnosis of injection-site sarcoma is confirmed, staging is necessary to determine if the cancer has spread. Your vet will recommend:
Treating feline injection-site sarcoma requires an aggressive, multi-modal approach. Because these tumors are highly invasive, single-therapy options rarely achieve long-term control.
Aggressive surgical excision is the cornerstone of FISS treatment. Because the tumor cells extend far beyond the visible lump, surgeons must plan for exceptionally wide margins—often removing 3 to 5 centimeters of healthy-looking tissue around the tumor, as well as the underlying muscle fascia or bone. If the tumor is located on a limb, amputation of the affected leg is often recommended to achieve complete removal.
Chemotherapy is frequently used alongside surgery to slow down tumor recurrence and target microscopic cancer cells that may have spread elsewhere in the body.
The prognosis for cats diagnosed with feline injection-site sarcoma is generally guarded. Because these tumors are highly aggressive and locally invasive, achieving a permanent cure is difficult.
If the tumor is detected when it is very small (less than 2 centimeters in diameter) and undergoes immediate, highly aggressive surgical excision by a veterinary surgeon, long-term remission or even a cure is possible. However, if the surgical margins are incomplete, local recurrence is extremely common. Recurrent tumors are much harder to treat and are associated with significantly shorter survival times.
Metastasis (the spread of cancer to other parts of the body) is a major concern. The metastasis rate is high, reaching up to 50% to 70% in recurrent cases. Furthermore, up to 20% of cats already have pulmonary metastasis (spread to the lungs) at the time of their initial presentation to the vet. Managing a cat with advanced FISS focuses on maintaining quality of life, managing pain, and using palliative therapies to keep the cat comfortable for as long as possible.
While you cannot completely eliminate the risk of feline injection-site sarcoma, you can take active steps to minimize the risk and ensure early detection. The most effective tool for owners and veterinarians is the "3-2-1 Rule" for post-vaccination monitoring.
As outlined in the feline vaccination guidelines:
"The Task Force recommends that veterinarians and owners monitor the vaccination site for swelling or lumps using the ' 3-2-1 ' rule. Biopsy of any mass present is warranted if it (1) remains present 3 months after vaccination, (2) is larger than 2 cm in diameter, or (3) is increasing in size 1 month after vaccination. 1,55 It is recommended to obtain an incisional bi"
By strictly adhering to this rule, you can catch potential sarcomas when they are still small and highly treatable.
Additionally, modern veterinary medicine has adapted injection protocols to improve safety. Veterinarians now administer vaccines lower on the limbs (such as the lower rear legs or tail) rather than in the scruff of the neck (the interscapular region). If a sarcoma does develop at a lower limb site, a complete surgical cure via limb amputation is far more achievable than if the tumor develops on the trunk or neck.
You should contact your veterinarian immediately if you notice any new lump or bump on your cat, particularly if it is located in an area where your cat has previously received an injection.
Watch for these specific warning signs:
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
Feline injection-site sarcoma (FISS) is a rare but highly aggressive cancer that can develop at injection sites in cats. Learn how to spot the signs using the "3-2-1 rule" and understand the diagnostic and treatment options available.
rapidly growing soft tissue mass、swelling or lumps、localised inflammatory reaction、pulmonary metastatic lesions
incisional biopsy、surgical biopsy、Abdominal ultrasound、CT、Thoracic radiographs、fine-needle aspirate (FNA)
Treatment must be prescribed by a licensed veterinarian based on your pet. Specific drug doses are intentionally not shown here.
This article is for general education and is not a substitute for professional veterinary advice. If your pet is unwell, please consult a veterinarian.
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