Fibrosarcomas in Cats

Causes, Treatment, and Prevention

cat laying down


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If you notice a lump on your cat, especially around a vaccination site, a fibrosarcoma may be to blame. Fibrosarcomas are cancerous tumors that develop on the soft tissue under a cat's skin. The lump will likely be located on the hipback, or shoulder blades and will be hard to the touch. In the beginning, the lump won't be painful, but if untreated and metastasized, your cat will become very sick. The cause of fibrosarcomas isn't entirely clear, but the inflammation from vaccinations is suspected to contribute to the development of the tumor. Your vet will diagnose a fibrosarcoma using a combination of tests, depending on the progression of your cat's tumor. Treatment usually involves removalradiation, and limb amputation when possible. The prognosis for a cat with a fibrosarcoma depends on the advancement of the growth, but swift treatment will extend your cat's life.

What Are Fibrosarcomas?

A fibrosarcoma is a cancerous, soft tissue tumor that can occur in cats. They usually form within connective tissue, cartilage, and fat under a cat's skin. The term "fibrosarcoma" accounts for various cancers but refers to any tumor that grows in soft tissue. Most fibrosarcomas are very invasive in local tissues, but the risk of spread to other organs is relatively low. Even high-grade, advanced tumors metastasize in only 25% of cats.

These tumors can grow quickly but, more often than not, are slow to develop. Fibrosarcomas often form at a place on your cat's body where it has received a vaccination. Though the explanation of the correlation isn't entirely clear, studies also show that in some cats, the process of receiving any substance into the skin is enough to induce inflammation related to the development of a fibrosarcoma.

Symptoms of Fibrosarcomas in Cats

The clearest indication of a fibrosarcoma in a cat is the presence of a lump below the skin. Lumps can vary in presentation and are sometimes accompanied by other symptoms.


  • Lump under the skin
  • Oral bleeding
  • Systemic symptoms

Lump Under the Skin

A lump under your cat's skin is the best indicator of a possible fibrosarcoma. A fibrosarcoma lump is firm and feels fixed to the tissue below the skin. The mass is likely not painful to your cat but can become infected and ulcerated if untreated. These lumps commonly occur at vaccine sites, so pay attention to growths at areas on the cat's body where vaccines have been administered, such as shoulder blades or hips.

Oral Bleeding

Sometimes, fibrosarcomas can develop in a cat's oral tissue, and the masses can break open and bleed. There will usually be other accompanying symptoms to an oral fibrosarcoma, such as tooth loss, excessive drooling, and inability to eat.

Systemic Symptoms

If your cat's fibrosarcoma has metastasized or is untreated, your cat will likely exhibit systemic symptoms beyond the lump, such as lethargy, decreased appetite, difficulty breathing, and pain. Without the presence of a lump, these symptoms are not necessarily related to cancer.

Causes of Fibrosarcomas

There are several suspected causes of fibrosarcomas, but there is usually no single explanation, like all cancers.

  • Vaccinations: The administration of a vaccine can be correlated to the development of a fibrosarcoma in your cat.  These tumors may not appear for years but are often at the vaccine site. It is thought that the local inflammation caused by the vaccine or the adjuvant in the vaccine can contribute to the formation of fibrosarcomas, but it is still not fully understood. Despite the correlation, the risks of not vaccinating your cat are higher than those of developing a vaccine site-related fibrosarcoma. 
  • Foreign bodies: Materials lodged under a cat's skin, such as a microchip or shard of plastic or glass in a wound, can cause inflammation that eventually results in a fibrosarcoma.
  • Feline sarcoma virus: Feline sarcoma virus (FeSV) is a rare hybrid virus that can cause an extremely fast-growing type of fibrosarcoma. Typically, this virus occurs in young cats that have also contracted Feline Leukemia Virus (FeLV).

Diagnosing Fibrosarcomas in Cats

To diagnose your cat with a fibrosarcoma, your vet will perform a physical exam and administer tests like chest x-rays, bloodwork, CT scans, and biopsies. Typically, the diagnosis begins with a fine needle aspirate, when a needle is inserted into the mass to obtain a cell sample for evaluation. Depending on the stage of the fibrosarcoma, your vet will determine the best diagnostic plan for your cat.


Treatment for fibrosarcoma and other soft tissue cancers usually necessitates surgery and chemotherapy. Mass removal surgery isn't always effective in removing cancer entirely because of the extent of the invasiveness of the tumor. Often, amputation is the best method for guaranteeing the complete elimination of cancer. Even when surgery is effective, some vets recommend proceeding with chemotherapy for additional protection. If the fibrosarcoma has spread, it will be much more challenging to treat.

Prognosis for Cats With Fibrosarcomas

The prognosis for cats with fibrosarcoma varies based on the stage and spread of the mass. Typically, fibrosarcomas are difficult to treat, and a cure is unlikely. Like all cancer treatments, the earlier a tumor is detected and treated, the longer your cat is likely to survive. Without complete removal of the fibrosarcoma with surgery and radiation therapy, it is very likely to recur within a year. However, if the tumor is treated effectively, there is a 50% chance your cat will survive for two years.

How to Prevent Fibrosarcomas

Because of the correlation between vaccinations and fibrosarcoma, it's beneficial to avoid over-vaccinating your cat and, when possible, elect for your cat to use oral medications and supplements instead of injections. Following your veterinarian's recommendations for vaccines will help limit the potential of injection inflammation that could lead to fibrosarcomas. However, you should not avoid vaccinating your cat in general.

Some owners choose not to put microchips in their cats to avoid inflammation that may lead to a fibrosarcoma. There are alternatives to microchips like GPS tracking collars, but a microchip is still the most reliable way to monitor your cat.

While not preventative of a fibrosarcoma, some vets opt to administer vaccines on a cat's limbs rather than back or hips. This allows for the option of amputation if a fibrosarcoma does occur.

  • Are fibrosarcomas caused by vaccinations common?

    When a fibrosarcoma occurs, it is usually at a vaccine site. The relationship between vaccines and fibrosarcomas is not entirely understood, but there are steps you can take to lower the risk of a tumor developing.

  • What does a fibrosarcoma lump feel like?

    A fibrosarcoma lump will be hard to the touch and immovable. Touching the lump won't hurt your cat, but it will become painful if left untreated.

  • What is the best treatment for a fibrosarcoma?

    Amputation is the best way to ensure that cancer doesn't spread. Ideally, the fibrosarcoma will be located on a limb so that it can be amputated; otherwise, your vet will use a combination of removal and chemotherapy. 

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Cat Fibrosarcoma. Long Island Veterinary Society.

  2. Vaccines and sarcomas: A concern for cat owners. AVMA.
  3. Tumors of the Skin in Cats. Veterinary Manual.