고양이 주사 부위 육종 자주 묻는 질문 Q&A — 보호자가 꼭 알아야 할 핵심

Why You Shouldn’t Ignore Lumps at Vaccination Sites — Feline Injection-Site Sarcoma Q&A

Oncology/CancerQ&AMeongsiljang Veterinary Advisory Board

Feline injection-site sarcomas are rare malignant tumors that can develop after vaccinations. We’ve compiled the key information pet owners need to know in a Q&A format.

What is a feline injection-site sarcoma?

A close-up photo of a hard tumor on a cat's shoulder area

Feline injection-site sarcomas are relatively rare malignant tumors that can develop in the subcutaneous tissue after vaccinations or injections. They are believed to arise when persistent inflammation at the injection site triggers abnormal proliferation of fibrous tissue, with onset varying widely from several months to several years. One study reported an average onset of approximately 26 months after rabies vaccination and about 11 months after feline leukemia virus (FeLV) vaccination. Initially, the mass is small, firm, and painless, but it may gradually enlarge, causing the skin to become red, ulcerated, or bleed. Early detection improves treatment outcomes, so it is important to consult your veterinarian if a lump at an injection site persists, fails to resolve, or continues to grow.

Why do sarcomas develop at injection sites?

The exact cause of injection-site sarcomas remains unclear. However, it is believed that residual foreign material or antigens at the injection site trigger chronic inflammation and immune responses, which in turn provoke an abnormal healing response in connective tissue, potentially leading to tumor development in some cases. Inactivated vaccines containing adjuvants are specifically designed to induce a stronger inflammatory response to stimulate immunity, and they have been reported to be more frequently associated with soft tissue sarcomas than other types of vaccines. Aluminum adjuvants have even been identified in tumor biopsies. Because this persistent inflammation can lead to abnormal cell proliferation, careful consideration of vaccine type and injection site selection is essential.

Diagram illustrating the mechanism of immune responses and abnormal cell proliferation at injection sites in cats

What are the main symptoms?

The primary sign of a feline injection-site sarcoma is a firm, relatively well-defined lump that doesn’t move easily, typically felt under the skin at the injection site. Initially small and painless, it may gradually enlarge, causing the overlying skin to become red, ulcerated, or bleed. Your cat might frequently lick or scratch the area, and as the tumor grows, it can invade surrounding tissues, limiting mobility. Because this type of tumor grows aggressively locally and spreads rapidly along the fascia, any lump at an injection site that persists or continues to enlarge should raise suspicion for malignancy. Therefore, regularly checking injection sites is essential.

Signs that require immediate veterinary care

If a lump at the injection site persists for an extended period, gradually enlarges over time, or is accompanied by skin redness, ulceration, or bleeding, please seek veterinary care immediately. These tumors can be locally aggressive and may be malignant, making early diagnosis and treatment essential.

How is it diagnosed?

If a suspicious tumor is found, a biopsy is essential. A biopsy accurately determines the nature of the tumor and whether it is malignant. Imaging tests are also important: CT or MRI scans help evaluate the tumor’s size, depth, and extent of invasion into surrounding tissues, aiding in surgical and radiation therapy planning. Abdominal ultrasound and chest radiographs are used to check for metastasis to the lymph nodes or lungs. Since this type of tumor primarily metastasizes to the lungs, chest imaging is particularly crucial at the time of diagnosis. Without an accurate diagnosis, a treatment plan cannot be formulated, so it is important to consult with your veterinarian to determine the appropriate diagnostic tests and respond promptly.

A veterinarian collecting cells from a cat's tumor.

What are the treatment options?

Treatment depends on the tumor’s size, location, and extent of invasion and metastasis. The most recommended approach is wide surgical excision with adequate margins (for example, approximately 5 cm laterally and deep enough to include two layers of fascia). Because incomplete removal of this tumor type carries a high risk of local recurrence, postoperative radiation therapy may be added in areas where achieving deep and wide margins is difficult, to help ensure long-term tumor control. Chemotherapy may also be added in certain cases. Early detection is crucial, as treatment becomes more challenging once the tumor grows large or metastasizes. Regular check-ups and monitoring before and after surgery are essential. A treatment plan should always be developed in consultation with your veterinarian to tailor it to your pet’s individual needs.

How can it be prevented?

To prevent adverse reactions, it’s best to consult with your veterinarian and administer only the vaccines that are truly necessary. If possible, consider using non-adjuvanted vaccines instead of those containing adjuvants, as they are expected to cause less inflammation. It is also recommended to administer vaccines in low-risk areas, such as the distal limbs or tail, where surgical intervention is easier if complications arise, rather than in high-risk areas like the interscapular region, where tumors can be difficult to fully excise. Additionally, removing refrigerated vaccines about 15 minutes before administration to allow them to reach room temperature may help reduce risks. After vaccination, closely monitor the injection site and seek veterinary care immediately if any abnormal signs appear.

Step-by-step comparison of managing injection-site sarcomas

Item	Symptom	Management	Prognosis
Early detection stage	Small, firm, poorly movable mass, no pain	Regularly observe the injection site, record changes	Relatively favorable as a localized tumor when detected early
Persistent/growth stage	Mass does not disappear and gradually enlarges, skin changes	Confirm malignancy with a tissue biopsy, consider wide surgery	Depends on early diagnosis and whether complete excision is achieved
Advanced/metastatic stage	Ulceration/bleeding, invasion of surrounding tissue, suspected metastasis to lungs, etc.	Wide surgery combined with radiation/chemotherapy	Poor prognosis with metastasis (metastasis reported in 25–70%)

The stages are a general classification based on the tumor's clinical features (size, growth, invasion, and presence of metastasis). Early detection and complete surgery with adequate margins have a major impact on prognosis.

Caution: Key points for injection site care

Frequently touching or scratching the injection site can worsen inflammation. If you suspect a tumor, never attempt home treatment; always consult your veterinarian. After the injection, keep a close eye on the area, and if a lump persists or grows larger, seek an examination right away.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Are injection-site sarcomas common in cats?

No, it's a very rare disease. In the United States, it's reported in about 1 to 4 out of every 10,000 vaccinated cats, and in the UK, in about 1 out of every 10,000 to 20,000 cats, indicating a low incidence rate. However, because it can be serious when it does occur, early detection is crucial.

Do all vaccines cause sarcomas at the injection site?

Not all vaccines are the same. Inactivated vaccines containing adjuvants, such as aluminum, are known to trigger stronger inflammatory responses and carry a relatively higher risk. Vaccines without adjuvants are expected to have a lower risk.

Can injection-site sarcoma be fatal in cats?

Yes, this tumor can primarily metastasize to the lungs, with reported metastasis rates ranging from 25% to 70%. Once metastasis occurs, treatment becomes more challenging and can be life-threatening, making early detection and appropriate treatment crucial for survival.

How can you check the injection site?

Gently press the injection site with your finger. If you feel a firm, immobile lump, or if the lump persists for an extended period or gradually grows larger, consult your veterinarian.

If a sarcoma develops at the injection site, can my pet still receive vaccines?

Whether additional injections are needed after the tumor has been completely removed should be determined by your veterinarian. If further injections are necessary, it is recommended to choose an injection site that is different from the previous one and located in an area where surgical intervention would be easier if any complications arise.

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References

[1] Hartmann K, Day MJ, Thiry E, et al. Feline injection-site sarcoma: ABCD guidelines on prevention and management. J Feline Med Surg 2015;17(7):606-13.

[2] Plumb's Veterinary Drug Handbook, 9th Edition. Wiley-Blackwell, 2021.

[3] American Association of Feline Practitioners (AAFP). Feline Vaccination Guidelines. 2023.