Feline injection site sarcoma (FISS) is a concerning condition we occasionally encounter in feline oncology. This rare and sometimes aggressive form of cancer can pose a significant challenge for both veterinarians and cat owners alike. In this article, we will delve into the various aspects of injection site sarcoma, including its types, signs and symptoms, diagnostic methods, treatment options, and prognosis.

What is Injection Site Sarcoma?

Injection site sarcoma, also known as vaccine-associated sarcoma, is a type of cancer that can develop at the site of a previous injection in cats. While vaccines are crucial for preventing numerous diseases, only a small number of cats (1 in 10,000-30,000 cats) may develop sarcomas at the site of injection. Locally, these sarcomas are highly invasive and have the potential to spread to other parts of the body, making early detection and intervention crucial.

Can Injection Site Sarcoma Be Prevented?

Veterinarians are researching ways to prevent and/or reduce the risk of injection site sarcoma in cats. Today, several strategies are being used to help reduce the chances of developing ISS including a combination of cautious vaccination practices and vigilant monitoring. Generally speaking, cats should be vaccinated only as often as absolutely necessary, considering the cat’s realistic exposure to disease in combination with local regulations regarding vaccination.

Current recommendations to reduce the risk of ISS include:

Choosing Three-Year Vaccine Protocol vs. Yearly Vaccine Protocol
A significant shift in feline vaccination protocol is the introduction of a three-year vaccine protocol. Instead of traditional annual vaccines, cats can now be protected against certain diseases for up to three years with a single vaccination. This approach reduces the overall number of injections a cat receives over its lifetime, thereby minimizing the potential for injection site inflammation and subsequent sarcoma development. Three-year vaccines are available for many common feline diseases, such as feline viral rhinotracheitis, calicivirus, panleukopenia (FVRCP), and rabies.

Using Non-Adjuvanted Vaccines
Non-adjuvanted vaccines are associated with a lower risk of sarcoma development. An adjuvant is a component used in vaccines to enhance the immune response in the recipient. Simply put, adjuvants aid in improving the effectiveness of vaccines but may also increase the chances of developing a tumor. Discuss vaccine options with your veterinarian.

Location of Vaccination
While we currently cannot prevent all vaccine-associated sarcomas, we can vaccinate cats in areas that are more amenable to treatment. Vaccination in the lower aspect of the hindlimb or in the tail lends itself to possibly more successful treatment options in the event that a tumor develops.

Monitoring Lumps Regularly
Regularly check your cat for any lumps, especially at injection sites. If you notice anything unusual, seek veterinary attention promptly.

Types of Injection Site Sarcoma

There are several variations of injection site sarcoma in cats, each with unique characteristics. These variations can include differences in tumor location, growth patterns, and aggressiveness. Some types of injection site sarcoma may develop near the site of a previous vaccination, while others may arise in response to other types of medications administered by injection. Feline injection site sarcomas require careful monitoring and timely veterinary intervention for proper management and treatment. The following are the various types of injection site sarcomas:

  • Fibrosarcoma: This is the most prevalent type of injection site sarcoma. It originates in the fibrous connective tissue around the injection site. Fibrosarcoma tends to be slow-growing initially but can become aggressive over time.
  • Malignant Fibrous Histiocytomas (or Fibroblastic Myxosarcoma): This type of sarcoma is characterized by a mixture of fibroblasts found in connective tissue and histiocytes. Malignant fibrous histiocytoma can be more locally invasive than fibrosarcoma and may require more aggressive treatment approaches.
  • Osteosarcoma: While less common, osteosarcomas can develop at injection sites. These tumors arise from boney tissue and tend to be quite aggressive.
  • Chondrosarcoma: A rare variation, chondrosarcoma, originates in cartilage tissue. It can be more challenging to diagnose accurately due to the location and characteristics.

Signs & Symptoms of Feline Injection Site Sarcoma

Identifying the signs and symptoms of injection site sarcoma in its early stages is vital for successful treatment so paying close attention is critical. Some common signs and symptoms of injection site sarcoma in cats include:

  • A palpable lump: A firm lump at from pea-sized to two centimeters or more in size may develop at the injection site, and may persist for three months or longer. Additionally, lumps that continue to increase in size are concerning.
  • Swelling or inflammation: The area around the injection site might become swollen, irritated, red or inflamed. When swelling and inflammation persist, your cat should be examined.
  • Pain or discomfort: Cats may exhibit signs of pain, discomfort, or general sensitivity when the injection site is touched. Again, if sensitivity persists, your cat should see their veterinarian.
  • Limping or lameness: If the tumor affects bone tissue, the cat may limp or show signs of lameness.
  • Changes in behavior: Cats might become more withdrawn, lethargic, or exhibit changes in eating habits. Cats tend to hide pain and discomfort, so changes may be very subtle.
  • If the tumor has progressed or has spread to other parts of the body (metastasized) cats may also experience systemic illness including:
    • Decreased appetite
    • Lethargy
    • Weight loss
    • Increased respiratory rate

These symptoms are not exclusive to injection site sarcoma and can be caused by a wide variety of other conditions as well. However, if you notice any of these signs, it’s important to have your cat examined by a veterinarian promptly to determine the underlying cause of the symptom.

Diagnosis & Staging of Injection Site Sarcoma

Diagnosing injection site sarcoma involves a comprehensive veterinary evaluation of your cat including a thorough physical examination and diagnostic work-up examining blood and urine samples to start. Advanced imaging (like X-ray, MRI, or CT) is often employed. Cytology or biopsy will be necessary for a diagnosis. When assessing a potential injection site sarcoma, your veterinarian will perform all of the necessary tests to ensure an accurate diagnosis and establish an appropriate treatment plan.

Here’s what to expect:

Physical Examination: Your veterinarian will conduct a comprehensive examination of the lump, paying close attention to its size, location, and distinct characteristics.

Imaging: To gain a deeper understanding of the tumor and potential invasion into surrounding tissues including any metastasis, imaging will likely be recommended. Advanced imaging techniques, like X-ray, MRI, and CT scans, provide invaluable insights into the extent of the disease and aid in formulating an effective treatment strategy.

Biopsy: A crucial step in the diagnostic process is the collection of a tissue sample from the lump for histopathological analysis. A veterinary pathologist will examine the tissue sample under a microscope so that the presence of cancer can be confirmed as well as the level of aggressiveness. Further, this analysis will help in determining the specific type of sarcoma and the tumor grade.

Performing a thorough assessment of your cat to confirm the diagnosis of injection site sarcoma is critical. This evaluation will serve as a valuable guide in formulating a targeted treatment plan that maximizes the chance of a successful outcome.

Treatment Options for Injection Site Sarcoma

Treating injection site sarcoma in cats can be complex and many times requires a multidisciplinary approach. Treatment options include:

Surgical removal of the tumor including wide margins is the primary treatment for injection site sarcoma. However, achieving clean margins can be challenging due to the invasiveness of these tumors. At times, amputation can be performed in order to obtain clean margins. When clean margins are difficult to achieve, other treatments post-surgery can help.

Radiation Therapy
Radiation therapy is often used to treat injection site sarcoma in cats. Radiation can be delivered post-surgery to target any remaining microscopic cancer cells and reduce the risk of local recurrence. Radiation therapy can also be used as the primary treatment for FISS in cases where surgery isn’t feasible due to the location of the tumor in the form of stereotactic radiation therapy (SRT/SRS) or conventionally fractionated radiation therapy (CFRT). With SRT, a patient may only need 1-3 doses of radiation total.

While chemotherapy has limited effectiveness against sarcomas, it may be used to attempt to slow metastatic disease or as part of a multimodal treatment plan.

Emerging treatments like immunotherapy aim to stimulate the cat’s immune system to recognize and attack cancer cells. While still being researched, these therapies hold promise for improving outcomes.

Prognosis for Cats with Injection Site Sarcoma

The prognosis for cats diagnosed with injection site sarcoma varies based on factors such as tumor type, size, location, and responsiveness to treatment. Achieving complete surgical removal with wide margins is associated with a much better prognosis than if clean margins cannot be obtained and no further therapy is elected. In these instances, in general the tumor tends to recur within a matter of only months. Even with aggressive treatment clean margins or a multimodality approach, there is a risk of local recurrence or metastasis to other parts of the body. Thankfully, many cats will respond favorably to radiation therapy, giving cats prolonged disease-free times.

Regular follow-up visits with your veterinarian are essential to monitor your cat’s progress and detect any signs of recurrence or metastasis. Keep in mind that each cat’s response to treatment is unique, and your veterinary team will work closely with you to make informed decisions about the best course of action.

Injection site sarcoma in cats is a complex and challenging condition that requires early detection, accurate diagnosis, and customized treatment approaches. While the prognosis can be guarded, advancements in veterinary oncology, including radiation therapy, are continually improving our ability to manage and treat injection site tumors. If you suspect your cat might have an injection site sarcoma, do not hesitate to consult with your veterinarian and have your cat examined promptly. Being proactive can make a significant difference in your cat’s quality of life and overall outcome.

Additionally, we encourage cat owners to reach out to our Pet Advocate team at 833-PET-HERO (833-738-4376) as a completely complimentary resource. Our highly trained team can help you understand an injection site sarcoma diagnosis and provide you with the information you need to help make the best treatment decision for your cat.

Types of Cancers In Cats

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