Cat Injection Site Lumps: Could It Be Fibrosarcoma? — Treatment and Prognosis by Location

Oncology/CancerDiseasesMeongsiljang Veterinary Advisory Board

Feline fibrosarcoma is a malignant tumor that originates in the skin and subcutaneous connective tissue. Treatment options and prognosis vary significantly depending on the tumor’s location and whether clear surgical margins can be achieved.

What Is Feline Fibrosarcoma?

A veterinarian palpating a subcutaneous mass on a cat's shoulder.

Feline fibrosarcoma is a malignant soft tissue sarcoma that arises from fibroblasts in the skin and subcutaneous connective tissue. The most critical factor is the tumor’s location. Tumors that develop in areas frequently used for injections, such as between the shoulder blades, the flank, or the hind legs, are called feline injection-site sarcomas (FISS) and are significantly more invasive than those in other locations. If a lump is 1 cm or larger or has persisted for more than three months, immediate examination is necessary.

Causes and Pathogenesis

Fibrosarcomas do not have a single cause. General fibrosarcomas are classified as tumors that arise spontaneously in older animals, while feline injection-site sarcomas (FISS) are known to develop when local irritants such as vaccines, repeated injections, or microchips cause chronic inflammation, leading to the malignant transformation of fibroblasts. In fact, cases of feline fibrosarcoma occurring adjacent to microchip implantation sites have been reported. The time it takes for these tumors to develop varies greatly among individuals, making it difficult to define a uniform incubation period. Once they appear, they tend to grow invasively, spreading along surrounding muscles and fascia. Therefore, it is important to consider both the tumor’s location and its pattern of progression.

Check for These Signs

Early-stage fibrosarcomas can look like simple lumps, making them easy to miss. Here are the signs you can check for yourself: - Size: A subcutaneous mass larger than 1 cm in diameter - Persistence: A lump that remains unchanged for more than three months or gradually grows - Location: Areas where injections are frequently given, such as between the shoulder blades, the sides, and the inner thighs - Surface: Initially smooth, but later developing ulceration or discharge - Fixation: Feels firmly anchored beneath the skin and does not move easily If you notice any of these signs, do not delay—seek a veterinary examination right away.

A pet owner checking for lumps on their cat’s side at home

When to Visit the Vet Immediately

Lumps larger than 2 cm in diameter, lumps that do not disappear after 3 months, and lumps at vaccination sites that persist for more than a month all fall under the "1·2·3 rule." In these cases, instead of simple observation, rapid differentiation through fine-needle aspiration cytology or incisional biopsy is necessary. FISS may appear small, but it often penetrates deeply along the fascia.

How Is It Diagnosed?

Diagnosing fibrosarcoma is a step-by-step process. First, we palpate the lesion to assess its location, size, and mobility, then perform a fine-needle aspiration cytology to evaluate the likelihood of malignancy. However, since cytology alone often cannot provide a definitive diagnosis, we conduct an incisional biopsy or core needle biopsy to determine the histological grade. Before planning surgery, it is essential to perform a CT or MRI scan to evaluate the depth of invasion and check for involvement of muscles or bones.

Treatment Options and Prognosis by Location

Item	Trunk (shoulder/flank)	Distal limb	Oral/facial
Primary treatment	Wide (radical) excision — 5 cm lateral margins and 2 fascial layers, or down to deep bone	Wide excision; consider limb amputation if margins are difficult to obtain	Wide excision ± radiation
Adjuvant treatment	Radiation ± doxorubicin combination for microscopic residual disease	Radiation if margins are insufficient	Radiation ± chemotherapy if margins are insufficient
Recurrence tendency	About 42% recurrence reported even after complete excision (Kobayashi 2002)	Limited evidence	Limited evidence
Survival data	About 51% overall survival in the radical excision group, with a median follow-up of about 600 days among surviving individuals (Romanelli 2008) — different from 'mean survival time'	Insufficient site-specific comparison data	Insufficient site-specific comparison data
Metastasis/staging	Metastasis is possible, so staging such as three-view thoracic radiographs is recommended	Regional lymph node aspiration and systemic staging	Mainly local invasion; imaging evaluation needed

Based on data from Romanelli et al. (2008), Phelps et al. (2011), and Kobayashi et al. (2002). Individual variation is large and evidence for site-specific comparison is limited, so the figures are for reference only.

Treatment — Surgical Excision Is Key

The key to treating fibrosarcoma is achieving wide surgical margins during the initial operation. For feline injection-site sarcomas (FISS), standard-of-care radical excision involves removing tissue with a 5 cm lateral margin from the tumor edge and extending deep to at least two layers of fascia or down to the underlying bone. Failure to secure adequate margins significantly increases the risk of recurrence; studies report recurrence rates of up to 42% even after complete resection. If microscopic residual disease is suspected post-surgery, radiation therapy is added to improve disease-free intervals and survival. In cases where metastasis is suspected or the tumor grade is high, doxorubicin-based chemotherapy may be combined with radiation. Because typical fibrosarcomas are relatively resistant to external beam radiation, securing wide surgical margins remains the most critical factor.

The veterinary surgical oncology team performing a wide excision surgery on a cat.

Post-Surgical Home Care Points

Because wide surgical excision leaves a large skin defect, the first two weeks after surgery are the most critical. - Wound care: Check the suture site daily and take photos to document any changes in discharge, redness, or swelling. - Activity restriction: Restrict jumping and running for at least 14 days, and keep the Elizabethan collar on 24 hours a day. - Diet: Provide a recovery diet with adequate protein. Contact the clinic if your pet’s appetite remains low for more than 24 hours. - Follow-up schedule: Sutures will be removed 10–14 days after surgery. Thereafter, chest X-rays should be performed every three months to check for lung metastasis. Recurrence typically begins at the same site within six months, so it is important to develop the habit of feeling that area every month.

Precautions to Prevent Recurrence

Once a cat has developed a fibrosarcoma, vaccinations should never be administered at the same site again. Future vaccines should be given in the distal part of a limb, following the "distal limb" principle, so that if the tumor recurs, amputation of just the affected limb is possible. It is also advisable to avoid repeatedly administering chronic irritants, such as microchips or steroid depot injections, to the same location. Follow-up examinations are recommended at least every three months for a minimum of two years.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Is feline fibrosarcoma always caused by vaccines?

Not necessarily. Spontaneous fibrosarcomas can also occur naturally in older cats. However, if a tumor develops between the shoulder blades or on the flank, there is a possibility it could be a feline injection-site sarcoma (FISS), so further evaluation is necessary.

Can it be treated with chemotherapy alone, without surgery?

It is not recommended. Wide surgical excision is the primary treatment for fibrosarcoma, while chemotherapy is used as an adjunctive therapy when surgical margins are inadequate, the tumor is high-grade, or metastasis is suspected. Radiation therapy may also be employed to address microscopic residual disease.

If it recurs, is surgery an option again?

While it is possible, incomplete removal significantly increases the risk of recurrence, making complete excision during a second surgery much more difficult. Therefore, securing adequate surgical margins during the initial operation is the most critical factor.

Can I skip my cat’s vaccinations because I’m worried they’ll be scary for her?

That’s actually more dangerous. Although the incidence of FISS is reported to be low, unvaccinated pets face a much higher risk of fatal infections such as panleukopenia and rabies. Instead, it’s recommended to minimize risks through proper care of the vaccination site and choosing non-adjuvanted vaccines.

What is the average life expectancy after surgery?

A study tracking FISS cats that underwent curative wide excision reported an overall survival rate of approximately 51%, with a median follow-up period of about 600 days for the survivors (Romanelli 2008). However, this 600-day figure represents the follow-up duration for surviving cats, not the average survival time, and outcomes can vary significantly depending on tumor location, grade, and surgical margin status.

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References

[1] Kudnig, S.T., Séguin, B., Veterinary Surgical Oncology, 2nd Edition, Wiley-Blackwell, 2022 — Chapter on Feline Injection-Site Sarcoma and Soft Tissue Sarcoma

[2] Phelps, H.A., et al., Radical excision with five-centimeter margins for treatment of feline injection-site sarcomas: 91 cases (1998–2002), JAVMA, 2011

[3] Romanelli, G., et al., Analysis of prognostic factors associated with injection-site sarcomas in cats: 57 cases (2001–2007), JAVMA, 2008

[4] Raskin, R.E., Meyer, D.J., Small Animal Cytologic Diagnosis: Canine and Feline Disease, 2nd Edition — Fibrosarcoma/Soft Tissue Sarcoma chapter