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

Due to medication injected into a cat, it is possible, though very rare, for a lump to form at the injection site. This is a tumor known as "Injection-site sarcoma." In this article, we will provide a detailed explanation regarding feline injection sites.

What is feline injection-site sarcoma?

A rare malignant tumor occurring after injection

Feline injection-site sarcoma is a malignant tumor that develops at the site of an injection. With an incidence rate of about 1 in 10,000 cats, it is considered quite rare. While the clear cause has not been fully elucidated, it is also called vaccine-associated sarcoma because it is associated with vaccinations.

Definition of "Injection-site Sarcoma (ISS)"

Injection-site sarcoma is a type of malignant tumor known as a sarcoma that occurs at the site where an injection was administered. Malignant tumors called fibrosarcomas account for the majority of these cases. They typically occur in middle-aged to older cats and form large lumps under the skin. While injection-site sarcoma can metastasize to other organs, such as the lungs, it is characterized by its tendency for local invasion, where the tumor extends roots into the surrounding tissue. Furthermore, even if surgically removed, recurrence is observed in about 80% of cases after six months.
Injection-site sarcoma tends to occur in middle-aged and older cats, and the risk increases if the cat has a history of receiving inactivated vaccines, such as the feline leukemia vaccine.

Which vaccines have a higher risk of occurrence?

It is believed to be associated with vaccines, specifically inactivated vaccines such as the feline leukemia vaccine and the rabies vaccine (cats are not subject to this vaccination in Japan). Besides vaccines, it can also occur due to injections used for medical treatment or even from wounds caused by fighting.

Causes of injection-site sarcoma

The impact of vaccine additives (adjuvants)

Injection-site sarcoma has been linked to vaccinations. Vaccines are categorized into live vaccines and inactivated vaccines, with injection-site sarcoma being observed with inactivated vaccines. Inactivated vaccines are made from pathogens that have had their infectious power removed, combined with components necessary to induce immunity. Since these components alone do not provide immunity, substances called adjuvants are added. It is believed that injection-site sarcoma is caused by these adjuvants triggering tumor formation.

Because it is thought that tumor formation is triggered not only by vaccines but also by subcutaneous inflammation, it can potentially occur from subcutaneous injections or fight wounds as well.

Symptoms of injection-site sarcoma and points for owners to note

Lumps after vaccination

After receiving a vaccine, temporary swelling can occur. This is because inflammation develops at the injection site. Lumps caused by normal inflammation typically disappear on their own within 6 to 8 weeks.

Danger signals when a lump does not disappear for a long time or grows in size

While lumps caused by inflammation disappear within two months, injection-site sarcoma does not disappear on its own and is characterized by its tendency to grow. Guidelines for tissue biopsies regarding injection-site sarcoma state that an examination is necessary if the following types of lumps are present:

• The lump has been present for more than 3 months after the injection
• It is 2 cm or more in diameter
• It continues to grow even after 1 month has passed since the injection

If you find a lump with these characteristics, an examination at a veterinary clinic is required immediately.

Points to check during palpation and timing for a clinic visit

Immediately after vaccination, there may be swelling or what feels like a lump, but this normally disappears naturally in about two months. However, if any of the characteristics mentioned above are present, there is a possibility of injection-site sarcoma, and you must visit a veterinary clinic.

After an injection, try to touch the area occasionally to check if a lump is present and ensure it is not growing.

Diagnosis and treatment of injection-site sarcoma

The importance of palpation and biopsy by a veterinarian

Because injection-site sarcoma spreads to surrounding tissue, it can sometimes be difficult to distinguish the boundary between the surrounding tissue and the lump. A full-body palpation is also necessary to confirm whether or not there is metastasis to the lymph nodes.

Since injection-site sarcoma can sometimes be misdiagnosed as other types of lumps, such as granulomas, a tissue biopsy—where the lump is removed and examined—is considered desirable as a general rule. A tissue biopsy also allows for confirmation of how far the tumor cells have penetrated into the surrounding tissue.

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

Because injection-site sarcoma can metastasize to the lungs, X-ray examinations are always performed to check for metastasis. Additionally, it may be necessary to perform imaging such as a CT scan to confirm how far the tumor cells have invaded and to check for metastasis to internal organs.

As it is a tumor prone to recurrence even after treatment, these examinations are conducted periodically.

Options and outcomes for surgery, radiation therapy, and chemotherapy

The primary treatment for injection-site sarcoma is surgical removal. However, injection-site sarcoma exhibits strong local invasion, rooting into surrounding tissues, which requires the removal of a fairly wide area including healthy tissue. In cases involving a limb, this may result in amputation. If the tumor is around 1 cm in diameter, there is a possibility of complete removal; however, there is a high probability of recurrence even after extensive surgical resection. If complete removal is not achieved, radiation therapy or chemotherapy (anticancer drugs) is also used.

There are reports comparing treatment outcomes regarding the time until recurrence for each therapy.

What you can do to prevent injection-site sarcoma

Consult your veterinarian about vaccination frequency and types

While vaccination frequency varies depending on the living environment and the type of vaccine, the "Vaccination Program for Dogs and Cats" recommends vaccination every three years. This is because frequent vaccinations in the same area are said to increase the risk of injection-site sarcoma. Since there are tests called vaccine antibody titer tests that check how much vaccine remains in the body, it may be beneficial to check antibody levels before vaccinating.

Currently, it is recommended that vaccines be administered in the legs. This is because, in the event that injection-site sarcoma develops, a life can potentially be saved by amputating the limb.

The importance of regular post-vaccination lump checks and early detection

Immediately after vaccination, the area may swell due to inflammation. While this lump normally disappears naturally within about two months, injection-site sarcoma will not disappear even after three months. Make it a habit to regularly touch the injection site, and if there is a lump that does not go away, consult a veterinary clinic immediately.

Summary

Injection-site sarcoma is a malignant tumor that occurs due to injections such as vaccines. Although it is a quite rare tumor, it easily spreads to surrounding tissue and is prone to recurrence. Early detection and early treatment are crucial for the treatment of this tumor. Strive for early detection by checking for lumps regularly.