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Fibrosarcoma in Cats - Causes, Symptoms and Treatment

Laura García Ortiz
By Laura García Ortiz, Veterinarian specialized in feline medicine. January 31, 2021
Fibrosarcoma in Cats - Causes, Symptoms and Treatment

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Also known as fibrous sarcoma in cats, feline fibrosarcoma is a type of soft tissue sarcoma which commonly appears at the injection site when hypodermic needles are used. As a relatively common type of cancer in cats, it accounts for 6 to 12% of feline tumors. It is a mesenchymal tumor, meaning it affects cells wh9ich grow into connective tissue, specifically fibroblasts. The neoplasm is very aggressive with a greater tendency for local recurrence. Initial symptoms appear as a small, but rapidly growing lump. Fortunately, metastasizing to other organs is relatively rare.

At AnimalWised, we look at the symptoms, causes and treatment of fibrosarcoma in cats. We investigate its link with vaccinations in cats, as well as other important health factors.


  1. What is fibrosarcoma in cats?
  2. Symptoms of fibrosarcoma in cats
  3. Causes of fibrosarcoma in cats
  4. Diagnosis of feline fibrosarcoma
  5. Treatment of fibrosarcoma in cats
  6. Prevention of fibrosarcoma in cats

What is fibrosarcoma in cats?

There are a range of soft tissue sarcomas, all types of malignant tumors caused by cancer. Fibrosarcoma is one in which fibroblast mesenchymal cells proliferate. These cells create the connective tissue which generate collagen and are involved in the wound healing process. It is known as a mesenchymal tumor since it originates in the mesenchymal cells. These form in the embryonic state of the feline to allow them to develop supporting tissues.

The appearance of feline fibrosarcoma is rounded, soft and/or solid in appearance. It can be uni- or multinodular (i.e. only one or several nodules). These nodules frequently adhere to nearby skin layers. It is not a painful or ulcerated neoplasm, except in its final stage. It is rare for fibrosarcoma in cats to metastasize to other organs or tissue, but recurrences of the tumor after surgical removal are common.

Symptoms of fibrosarcoma in cats

Feline fibrosarcoma can occur at any age, but it is most common in senior and middle-aged cats. The lump is initially very small and is usually first observed when the cat is being stroked. It will increase in size very rapidly.

Since it is attached to subcutaneous tissue and the underlying musculature, the fibrosarcoma can be relatively mobile. It is also possible it can infiltrate into surrounding structures. This means the space between the tumor and surrounding healthy tissue can be difficult to differentiate. For cats, the injection site is most commonly the interscapular area, i.e. the space between the cat's shoulders or neck. Large tumors can be damaged by trauma or ulceration. Our article on why a cat has a lump on their neck can explain other causes of such inflammation.

In the cases where metastasis does appear, it usually happens in the lung. For this reason, respiratory problems are likely to appear. These can be confused with other upper respiratory problems such as sinusitis.

Fibrosarcoma in Cats - Causes, Symptoms and Treatment - Symptoms of fibrosarcoma in cats

Causes of fibrosarcoma in cats

Although there is a link between vaccinations and fibrosarcoma, it is important to know that this complication is uncommon[1]. It can be a result of other injections, not just inoculations. The exact etiopathogenesis is unknown, but the following are considered causes of fibrosarcoma:

  • Vaccination: after vaccination at the site of inoculation with a hypodermic needle. Some cats develop a small inflammatory nodule at this point that disappears in about two to three weeks as a side effect of vaccination. If it does not go away, chronic inflammation can lead to the development of this tumor. This inflammation at the vaccine inoculation site occurs more frequently in vaccines with adjuvants, the components added to vaccines to improve their effectiveness. The vaccines that most frequently contain adjuvants are those for rabies and feline leukemia, therefore they are most associated with feline fibrosarcoma.
  • External agents: fibrosarcoma can be related to other external agents in the subcutaneous tissue, such as microchips, lufenuron or long-acting antibiotics.
  • Feline sarcoma virus: another less common cause is that the feline sarcoma virus, derived from the feline leukemia virus, which gives rise to this type of tumor.

Diagnosis of feline fibrosarcoma

Due to the possibility of fibrosarcoma in cats, the injection site needs to be monitored in the weeks following vaccination administration. If an abscess at the injection site appears, an ultrasound scan needs to be carried out as part of a differential diagnosis. Feline leukemia virus infection also needs to be tested for.

Cytology is of little use in diagnosis of fibrosarcoma in cats. If it is carried out, it will require a wedge biopsy and pathological study. This biopsy will usually only be performed in the presence of lumps more than 2 cm in width and in those which have been present three months after inoculation or have grown significantly after 1 month.

The histology of the biopsy will test for the important inflammatory component. it will also look for the proliferation of mononuclear cells, fibrosis and granulation. These types of soft tissue sarcoma in cats are characterized by high cellular mitosis (cell division) and frequent central necrosis (premature cell death). X-rays of the chest should also be carried out to assess whether metastasis has occurred in the lung or other areas.

Treatment of fibrosarcoma in cats

Fibrosarcoma treatment will depend on the size and location of the tumor. Whether there is metastasis will also affect the treatment. Possible treatments of fibrous sarcoma in cats includes:

  • Complete removal of the tumor: the main therapy will consist of the complete surgical removal of the tumor. This will likely include the removal of muscles and fascia adjacent to the tumor, due to its great infiltrative capacity. Surgical margins of at least 2 cm are required, ideally 3-5 cm lateral and deep to the tumor mass,. This may include the dorsal vertebral spinous processes and the dorsal edge of the scapula.
  • Radiotherapy: radiotherapy can be used in this type of feline cancer. The irradiated area will likely be the location where the tumor was before the incision and it should be done when healing has begun, a week or two after surgery. The efficacy of radiation therapy varies depending on the number of previous excisions, size before excision and quality of surgical excision.
  • Chemotherapy: chemotherapy can be 50-60% effective using carboplatin or doxorubicin. Non-vaccine associated feline fibrosarcomas have a lower response to chemotherapy at around 10-15%.

If there are metastases, aggressive surgery should not be performed.

Feline fibrosarcoma prognosis

The prognosis for feline fibrosarcoma is reserved, due to the high risk of recurrence. However, with a successful surgery, and with complementary chemotherapy and radiotherapy treatments, it is common for the cat to live several more years.

Fibrosarcoma in Cats - Causes, Symptoms and Treatment - Treatment of fibrosarcoma in cats

Prevention of fibrosarcoma in cats

The incidence of this disease in cats increases the more they are vaccinated. However, the incidence of diseases and their fatal consequences prevented by vaccination are much high than the risk of a fibrosarcoma tumor developing. For this reason, a suitable vaccination schedule is recommended for all cats.

To prevent difficulty in removing a tumor in the interscapular area, it is sometimes recommended cats are vaccinated in other areas. These include the extremities or the area behind the cat's ribs. If the tumor does appear at the vaccination site of a limb, the limb can be amputated or a better extirpation (removal) can be achieved within the necessary surgical margins.

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

If you want to read similar articles to Fibrosarcoma in Cats - Causes, Symptoms and Treatment, we recommend you visit our Other health problems category.


1. Saba, C. F. (2017). Vaccine-associated feline sarcoma: current perspectives, Veterinary Medicine (Auckland), 8, 13-20.


Gütgemann, F. Feline fibrosarcoma. Available at: https://www.zooplus.es/magazine/gatos/salud-del-gato-y-cuinados/fibrosarcoma-felino

Harvey, A., & Tasker, S. (Eds). (2014). Feline Medicine Manual. Ed. Sastre Molina, SL L ́Hospitalet de Llobregat, Barcelona, Spain.

Galán, A., & Villalobos, C. M. (2005). Feline fibrosarcoma associated with vaccines and therapeutic injections. Available at: https://www.portalveterinaria.com/animales-de-compania/articulos/16912/fibrosarcoma-felino-asociado-a-vacunas-e-inyecciones-terapeuticas.html

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Fibrosarcoma in Cats - Causes, Symptoms and Treatment