💉 Veterinary Guide to Injection Site Sarcoma in Cats: Prevention and Treatment Insights for 2025 🐾

By Dr. Duncan Houston, BVSc

🔍 What Is Injection Site Sarcoma?

Injection Site Sarcoma (ISS), also known as vaccine-associated sarcoma or vaxosarcoma, is a rare but serious type of cancer in cats. These tumors, most commonly fibrosarcomas, are locally aggressive and originate in areas where injections such as vaccines or medications are administered. They don’t typically spread early but grow deeply into local tissues, making treatment difficult and recurrence likely.

💡 How and Why Does It Occur?

While fibrosarcomas have existed long before injections were linked, a pattern emerged in the 1990s connecting vaccines—especially those containing adjuvants—to tumor formation in injection sites. Key observations include:

• 📈 Increased sarcoma rates in areas with mandatory rabies vaccination
• 📍 Tumor locations matching injection zones: shoulder blades, lower back, hind limbs
• 🧬 Presence of vaccine adjuvant material in tumor biopsies

🧬 The Role of Adjuvants and Inflammation

Adjuvants are added to killed virus vaccines (e.g., rabies and FeLV) to boost immune response. In some cats, the resulting inflammation becomes chronic and may initiate cancerous changes. The likelihood depends not just on the vaccine but also on the cat's genetic susceptibility.

📉 How Common Is It?

Thankfully, the incidence is low—about 1 in 1,000 to 1 in 10,000 injections. Considering the typical vaccination schedule, most cats receive around 45 injections in a lifetime, making this a rare complication. However, the seriousness of ISS warrants awareness and preventive steps.

🩺 Recognizing Signs Early

Lumps can form shortly after vaccination and are often benign. However, any lump should be monitored closely using the 3-2-1 Rule:

• 📅 3 months: If the lump is still present after 3 months, seek evaluation
• 📏 2 cm: Any lump over 2 cm in size warrants immediate attention
• 📈 1 month: If a lump is growing one month after discovery, biopsy it

💉 Should I Still Vaccinate My Cat?

Absolutely! Vaccinations save lives. With improved formulations and techniques, risks are now lower than ever. Steps to reduce ISS risk include:

• 🧬 Using non-adjuvanted or recombinant vaccines
• 📍 Administering injections in limbs or tail—areas easier to amputate if needed
• 📆 Tailoring vaccine protocols based on individual risk and lifestyle

🧪 Diagnosis and Staging

Diagnosis involves biopsy and imaging (X-rays or CT scans) to assess spread. Chest radiographs help identify metastasis. Tumor mapping is essential for surgical planning due to the tumor's “tentacle-like” growth.

🛠️ Treatment Options

1. 🚨 Surgery

The cornerstone of treatment. Requires wide margins—typically 2 inches in all directions. If the tumor is on a limb, amputation may be the most effective option.

2. 🔥 Radiation Therapy

Can be used pre- or post-surgery. Pre-surgical radiation can shrink the tumor; post-surgical radiation targets remaining cells. Also used palliatively when surgery isn’t possible.

3. 💊 Chemotherapy

Less commonly used but may be helpful depending on the tumor’s grade and whether other treatments are used.

⏳ What Happens Without Treatment?

Untreated tumors will continue to grow, eventually breaking through the skin, becoming infected, and severely affecting the cat’s quality of life. Without intervention, decline often occurs within six months.

📋 Summary of Prevention Strategies

• 💉 Use recombinant or non-adjuvanted vaccines when possible
• 📍 Administer injections in tails or lower limbs for safer surgical outcomes
• 🧪 Monitor all post-vaccine lumps using the 3-2-1 Rule
• 📆 Customize vaccination schedules based on lifestyle and risk

📱 Need Help Deciding on Vaccines or Noticed a Lump?

Talk to a vet today on AskAVet.com or via the Ask A Vet app! We're here to help you make informed choices and keep your furry friend safe and healthy. 🐾❤️