Feline Injection Site Sarcoma: Causes, Symptoms, and Prevention Explained by a Veterinarian

Although extremely rare, a lump may form at the injection site in cats due to the administered medication. This tumor is called "Injection Site Sarcoma." This article provides a detailed explanation of injection sites in cats.

What is Feline Injection Site Sarcoma?

A rare malignant tumor that occurs after injections

Feline injection site sarcoma is a malignant tumor that occurs at the site of an injection. It is a very rare tumor, with an occurrence rate of about 1 in 10,000 cats. The clear cause is not yet fully understood, but it is also called vaccine-associated sarcoma due to its connection with vaccination.

Definition of "Injection Site Sarcoma (ISS)"

Injection site sarcoma is a type of malignant tumor, specifically a sarcoma, that occurs at the site of an injection. Fibrosarcoma accounts for a large proportion of these malignant tumors. It develops in middle-aged and older cats, forming a large lump under the skin. Injection site sarcoma is a tumor that can metastasize to other organs like the lungs, but it is characterized by its tendency to invade surrounding tissues locally. Furthermore, even after surgical removal, recurrence is observed in about 80% of cases within 6 months.
Injection site sarcoma tends to occur in middle-aged and older cats, and the risk of developing it increases if the cat has received inactivated vaccines, such as the feline leukemia vaccine.

Which vaccines carry a higher risk of occurrence?

It is believed to be associated with vaccines, and inactivated vaccines, such as the feline leukemia vaccine and rabies vaccine (not applicable to cats in Japan), are thought to be the cause. In addition to vaccines, it can also occur from injections used for treatment or from fight wounds.

Causes of Injection Site Sarcoma

Effect of Vaccine Adjuvants

Injection site sarcoma has been linked to vaccination. There are live attenuated vaccines and inactivated vaccines, but injection site sarcoma is observed with inactivated vaccines. Inactivated vaccines are those where the pathogen's infectivity has been eliminated, and only the components necessary to induce immunity are included in the vaccine. Since these components alone do not induce immunity, a substance called an adjuvant is added. It is believed that this adjuvant causes tumorigenesis in injection site sarcoma.

It is thought that tumor formation can occur not only from vaccines but also from subcutaneous inflammation, so it can also occur from subcutaneous injections or fight wounds.

Symptoms of Vaccine-Associated Sarcoma and Points for Owners to Note

Lump after vaccination

After a vaccine injection, there may be temporary swelling. This is due to inflammation at the injection site. Lumps caused by normal inflammation typically resolve spontaneously within 6 to 8 weeks.

Warning signs if the lump persists for a long time or grows

Lumps caused by inflammation disappear within 2 months, but injection site sarcomas do not disappear spontaneously and typically grow. The guidelines for tissue biopsy of injection site sarcomas state that an examination is necessary if a lump has the following characteristics:

• Lump persists for more than 3 months after injection
• Diameter of 2 cm or more
• Grows more than 1 month after injection

If you find a lump with these characteristics, you should immediately have it examined by a veterinarian.

Key points for palpation check and timing for veterinary visit

Immediately after vaccination, the site may swell or form a lump, but this typically resolves naturally within about 2 months. However, if it meets the characteristics mentioned above, it could be an injection site sarcoma, and a veterinary visit is necessary.

After an injection, occasionally touch the area to check for a lump, and if one is present, monitor if it is growing.

Diagnosis and Treatment of Vaccine-Associated Sarcoma

Importance of veterinary palpation and biopsy

Injection site sarcoma spreads to surrounding tissues, so the boundary between the surrounding tissue and the lump can be unclear. Also, a full body palpation is necessary to check for lymph node metastasis.

Since injection site sarcoma can sometimes be misdiagnosed as other lumps like granulomas, tissue biopsy, which involves excising the lump for examination, is generally recommended. Tissue biopsy can also confirm how deeply the tumor cells have invaded the surrounding tissues.

Confirmation of tumor spread through X-rays, CT, and MRI

Injection site sarcoma can metastasize distantly to the lungs, so X-ray examination is always performed to check for metastasis. Additionally, CT scans and other imaging may be necessary to confirm how far the tumor cells have spread and if there is metastasis to internal organs.

Since this tumor is prone to recurrence even after treatment, these examinations are conducted regularly.

Treatment options and outcomes for surgery, radiation therapy, and chemotherapy

Surgical removal is the primary treatment for injection site sarcoma. However, injection site sarcoma exhibits strong local invasion into surrounding tissues, requiring a wide excision that includes a significant amount of normal tissue. If it occurs in a limb, amputation may be necessary. For tumors about 1 cm in diameter, complete removal might be possible, but recurrence is highly likely even with extensive excision. If complete removal is not achieved, radiation therapy or chemotherapy (anti-cancer drugs) may be combined.

There are reports comparing the treatment outcomes for each method in terms of time to recurrence.

What You Can Do to Prevent Vaccine-Associated Sarcoma

Discuss vaccine frequency and type with your veterinarian

The frequency of vaccination varies depending on the cat's living environment and the type of vaccine, but the "Canine and Feline Vaccination Program" recommends vaccination every three years. This is because frequent vaccination at the same site is believed to increase the risk of injection site sarcoma. There is also a vaccine antibody titer test that measures how much vaccine remains in the body, so it may be beneficial to check the antibody titer before vaccination.

Currently, it is recommended to administer vaccines in the leg. This is because, in the unlikely event of an injection site sarcoma, amputation of the leg can save the cat's life.

Importance of regular lump checks and early detection after vaccination

Immediately after vaccination, the injection site may swell due to inflammation. This lump typically resolves naturally within about two months, but an injection site sarcoma will not disappear even after three months. Regularly palpate the vaccination site, and if you find a persistent lump, consult a veterinarian immediately.

Summary

Injection site sarcoma is a malignant tumor that can occur due to injections, such as vaccines. It is a rare tumor, but it tends to spread to surrounding tissues and is prone to recurrence. Early detection and early treatment are crucial for tumor treatment. Regularly check for lumps to facilitate early detection.