Injection Site Sarcoma in Cats

Injection site sarcoma is a rare complication of vaccination and other types of injectable medications in cats. In addition to the ongoing presence of a mass around the injection site, specific symptoms may include

• Inflammation around the mass
• Sensitivity or avoidance of the area around the tumor
• Rapid growth around the area
• Ulceration
• Swelling of the limb around or below the mass

While this is a difficult disease with a poor prognosis, it is very rare. Despite the slight risk of developing injection site sarcoma, it is recommended to vaccinate cats as per current veterinary guidance according to lifestyle and regional considerations. Modern vaccines are designed to decrease the likelihood of sarcoma formation, and standardized injection locations facilitate both surgical removal and collection of information about sarcoma triggers. The diseases that are prevented by vaccination are more common and can be equally as damaging, including FeLV, FIV, and FIP.

Injection site carcinoma is a very rare feline condition, but is most often fatal. If caught early and in a location that is suited for wide surgical resection, the prognosis is best as it is a difficult type of tumor to treat.

Injection site sarcomas rarely metastasize, but they are still life-threatening because they are extremely locally invasive.

Approximately 80% of injection site tumors are specifically fibrosarcomas, other forms of tumors are possible and include;

• malignant fibrous histiocytoma
• osteosarcoma
• rhabdomyosarcoma
• liposarcoma
• chondrosarcoma
• undifferentiated sarcoma

Injection site sarcomas occur after an injection by a needle into the soft tissue of the skin. Vaccinations are the most common example of this but other injections including antibiotics and corticosteroids may induce tumor formation. Chronic, concentrated inflammation of the area around the injection appears to be the primary cause.

The exact cause of this response is unknown but likely has a genetic component to it. Some injections carry a higher likelihood of inducing the inflammation necessary (killed virus vaccines).

The main symptom is a mass larger than 2cm diameter around the area of injection. This can take anywhere from 2 months to 10 years to appear, with the median length of time occurring within 4 years of the injection.

Common injection sites are the back of the neck, between the shoulder blades, and on the lower leg. New protocols call for injections to be given only on the limbs, below the elbow or knee. In order to learn more about which injections are more likely to trigger sarcoma formation, protocols dictate administration of specific vaccines in certain limbs.

Diagnosis begins with a physical examination and medical history. The main diagnostic is a surgical biopsy or needle aspiration. Once sarcoma is confirmed, additional diagnostics may include

• Blood work
• Diagnostic imaging

Treatment is generally aggressive and involves wide surgical excision of the tumor where possible. This generally involves amputation of the affected limb. Surgery is often followed by radiation therapy to reduce the likelihood of recurrence.

If surgical removal is not possible, radiation can be used to shrink the tumor to attempt to facilitate a possible excision. The effectiveness of chemotherapy for these tumors is not well established but may be used to offer secondary assistance in treatment.

In addition to these, supportive care options are available:

• Analgesics and antibiotics for secondary infections of ulcerated tumors
• Palliative care

Injection site sarcoma is a difficult condition to treat and can establish deep roots in the tissue. The tumor can become infected and dangerous within 6 months of formation.

Aggressive and multifaceted treatment gives the highest chance at remission if it is in an area that can be amputated, but overall prognosis is not good. In cats with successful excision, the median survival time was 901 days and recurrence occurred in 14% of patients. In cases where both surgery and radiation were utilized, the median survival time was approximately 23 months.

It is not known what genetic factors or which specific vaccines or injections may trigger injection site sarcoma. There are steps that can be taken to reduce the risk of developing this type of tumor, including:

• Avoid excessive vaccinations
• Utilize live virus or recumbent virus vaccinations wherever possible
• Ensure all injections are done on the lower extremities to allow for the best chance of wide surgical excision
• If a reaction occurs, stop further vaccinations or injections

Injection site sarcoma is not contagious.

Injection site sarcoma is rare in cats.

• Radiation
• Chemotherapy
• Surgery
• Palliative care