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Medical Oncology: Feline Injection Site Sarcoma

Feline injection-site sarcoma arises from connective tissues. The most common injection-site sarcoma in cats is a fibrosarcoma. 

Regardless of underlying tissue type, all injection site sarcomas behave as locally aggressive tumors with a modest chance of spread to distant sites in the body.

Injection site sarcomas arise at sites where the cat previously received an injection. Tumors are caused by vaccines (feline leukemia virus and rabies vaccines), microchips, injections of long-acting antibiotics, long-acting glucocorticoids, lufenuron, and reaction to nonabsorbable suture. Cats may be genetically predisposed to developing injection site sarcomas, though the exact nature of this relationship is unclear.

Despite the association with vaccination, the risk of not vaccinating for certain diseases may be much higher than the risk of tumor development.  Vaccine protocols and injection methods continue to be evaluated. Cat owners should talk with their family veterinarian about which vaccines, including boosters, are recommended. Cats with a history of injection-site tumors should not receive future vaccinations.

What are the clinical signs?

Owners typically note a lump while petting their cat, or the tumor may be found incidentally on physical examination. Tumors can occur as early as 4 weeks, and as late as 10 years, post-vaccination. Guidelines for when to have a mass evaluated are: Persistence for greater than three months, if the mass is larger than two centimeters, or if the mass increases in size over one month. If metastasis (spread) is present, signs of systemic illness including lethargy, decreased appetite, vomiting, and increased respiratory rate and/or effort may be noted.

What diagnostics are performed?

The typical initial diagnostic step involves fine needle aspirate cytology, where a small needle is inserted into the mass to obtain a sample of cells for cytologic evaluation. Differentiating a sarcoma from a vaccine reaction or other collection of inflammatory cells is difficult and this form of sampling may be unsuccessful in achieving a definitive diagnosis. A surgical biopsy can be done in such cases. An incisional biopsy involves removing a small piece of tissue from the mass whereas excisional biopsy entails an attempt at removing the entire mass. Given the locally invasive nature of these tumors, advanced imaging such as contrast-enhanced CT or MRI is typically necessary to determine the extent of disease prior to an attempt at excisional biopsy. Additional diagnostics include blood tests to assess overall health and chest x-rays for the evaluation of disease spread. Regional lymph nodes should be palpated carefully and assessed cytologically if possible.

Learn more about Cancer Staging tests here.

Treatment options available and prognosis


Injection site sarcomas infiltrate extensively through surrounding tissues. Wide surgical resection is recommended given the high rate of local tumor recurrence with incomplete resection. Recurrence can occur even when complete surgical margins are obtained and may occur soon, or up to several years, after removal. Active surveillance of the surgery site is recommended to monitor for recurrence.

Radiation Therapy

Radiation therapy can be done prior to, or after, surgical removal of the tumor. Radiation therapy consists of daily treatments for several weeks. Radiation therapy is well tolerated by animals though brief anesthesia is required.  Radiation therapy combined with surgery has been shown to significantly improve survival over surgery alone.


Chemotherapy is recommended as part of a treatment plan for most injection site sarcomas. Spread to distant sites in the body occurs in up to 25% of cats.  This risk increases when the tumor regrows after repeated surgeries. Several chemotherapeutic regimens have been shown to have activity against injection-site sarcomas. Alternative options include immunotherapy with an IL-2 vaccine and electrochemotherapy.

The prognosis for cats with injection-site sarcomas depends on several variables. In general, these are difficult tumors to treat and cure is rare. Surgery alone can offer an overall survival of 12-16 months and the longest prognosis is given when an aggressive first surgery is done by a board-certified veterinary surgeon. When tumors are located along the limb, amputation can be the best option for long term tumor control. Combining surgery, chemotherapy, and radiation therapy increases the tumor-free interval to 18-24 months.  Cats with tumors that have grown back after repeated surgeries have a poorer prognosis. The best time to treat with chemotherapy and radiation therapy at the time of initial diagnosis, rather than after regrowth is noted.

What are the side effects?

Side effects depend on the treatment selected and the extent of disease and clinical signs. Surgery carries the risk of anesthesia, though this is minimal. Other risks include bleeding and complications from healing or infection. Side effects of chemotherapy are infrequent and most commonly include temporary gastrointestinal upset such as vomiting or diarrhea. Decreased appetite and lethargy may also occur. Radiation therapy includes the use of anesthesia during treatments and side effects include gastrointestinal upset (vomiting, diarrhea), local skin irritation, and potential interference with surgical healing. Your pet will be prescribed supportive medications for nausea (manifested by decreased appetite, or increased salivation, or drooling occur) and diarrhea for you to have on hand at home to use if necessary. It is best to be proactive with these medications and provide these as soon as signs are noted. Should you have any questions, your oncology team is available to assist.

Concerns of chemotherapy for my pet

Chemotherapy often carries a negative impression, especially with our understanding of chemotherapy in human medicine. Our approach to chemotherapy in veterinary medicine is focused on limiting severe side effects and providing increased quality of life. Chemotherapy in human medicine is provided with the intent to cure by using very high doses and increased side effects. As quality of life is imperative for our pets, doses are adjusted, and your pet is monitored to limit severe side effects. Hair loss is rare but cats tend to lose their whiskers. Though there is a slight risk of hospitalization in our pet population and mild gastrointestinal upset, the majority of pets tolerate therapy well. Should you have concerns during therapy, speak with your oncologist in order to develop a tailored plan for your pet.

How do I prepare?

We understand this is a difficult time and we are here to support you and your pet by providing the options and care necessary. Selecting a therapy is not binding and can be adjusted to you and your pet’s needs. During treatment sessions, you will be provided with updates and any recommendations depending on your pet’s response. Should any concerns arise, your oncology team will provide answers and help to guide you.

Navigating through my options

TreatmentPrognosis- (median)/
Duration of tumor
Treatment Schedule
SurgeryVaries with
aggressiveness of
procedure: 3 months to
2.5 years
CT scan followed by surgery
Radiation Therapy:
pre- or post-operative
For non-surgical tumors:
• Stereotactic
• Palliative
18-24 months

10 months
3 months
Varies between 14-20 daily treatments.
CT scan required.
3-5 consecutive treatments. CT scan
required. (Stereotactic)
4-6 daily or weekly treatments. CT scan
likely required. (Palliative)
Surgery and ChemotherapyMay be better than
surgery alone
Injectable chemotherapy: every 2-3 weeks
starting 10-14 days post-operatively
Metronomic chemotherapy: daily or every other day, at home
Surgery and Radiation

Therapy (pre- or post-
18-32 months pending
completeness of
Surgery, Radiation Therapy, and ChemotherapyUp to 2 years or moreCT scan, surgery, in some cases post-
operative CT, radiation therapy (10-14 days
postoperative), chemotherapy
Chemotherapy aloneControl 4 months,
Survival 8 months if
Injectable chemotherapy every 2-3 weeks
Feline IL 2 Immunotherapy
When combined with
surgery and radiation,
> 24 months
Series of 6 injections - schedule varies if
done pre-surgery vs. post-surgery. Tumor
should be 5 cm or less in size for optimal
Electrochemotherapy6 months1-3 treatments every other week post-

Getting started

Once you have determined the best therapeutic option for your pet, you may work with our oncology team to develop an appointment plan.


Patients undergoing treatments must have a scheduled appointment prior to arrival.

  • Schedule your appointments at reception upon check out
  • Drop-offs are requested between 7:30-8:30 am
  • Pickups are requested by 4:30 pm