Feline Injection Site Sarcomas
Injection site sarcomas (ISS, often referred to as vaccine-associated sarcomas) are locally aggressive tumors induced by injections. They occur in approximately 1 out of 10,000 vaccine injections. Although they do not occur with high frequency, it will be important for all practitioners who administer injections to cats to know how to prevent and manage this iatrogenic disease.
Key Points
- Biologic behavior
- Extremely locally invasive with tendrils or “fingerlike projections”
- Metastatic rate of 20-25%
- Occurs in 1 out of 10,000 vaccine injections
- Refer to a board certified surgeon for longer disease free intervals prior to recurrence
- Vaccinate as distally as possible on a limb to facilitate local treatment by amputation if ISS develops
Etiology
- Believed to be caused by inflammation associated with injections, particularly with the adjuvant in vaccines
- Rabies and FeLV vaccines historically have been most commonly implicated
- Other injections such as the upper respiratory and panleukopenia vaccine (FVRCP+/-C), lufenuron, penicillin, microchips, etc have also been implicated
Diagnostics
Fine needle aspirate cytology
- Often not definitive
Biopsy
- More accurate than FNA
- Compared to naturally occurring sarcomas, ISS histology has more necrosis, inflammation with lymphocytes and macrophages, and more mitotic figures
- Occasionally may see macrophages containing vaccine adjuvant
- 1, 2, 3 rule: biopsy is indicated if a vaccination site lump fits any of these criteria:
- Continues to grow after 1 month post vaccination
- Larger than 2 cm
- Still present after 3 months Thoracic radiographs
- To assess for pulmonary metastasis
CT or MRI
- To assess invasiveness for surgical planning
- External tumor is considered “tip of the iceberg” and is often several times larger than what is visible externally
Treatment and Prognosis
Surgery
- Radical excision with 3-5 cm lateral margins and 2 fascial planes deep is recommended
- Amputation if on a limb
- Often requires body wall resection with reconstruction or hemipelvectomy for proximal pelvic limb tumors
- Dorsal spinous process removal and partial scapulectomies often required for interscapular tumors
- Time to tumor recurrence
- About 2 months with conservative surgeries
- About 9 months with radical surgeries performed by referral surgeons
- About 16 months if surgical margins are “clean” or complete on histopathology
Radiation therapy
- Adjuvant treatment to surgery to prolong recurrence if margins are narrow or incomplete on histopathology
- Can be performed before or after surgery
Chemotherapy
- Adriamycin (doxorubicin) chemotherapy has up to a 50% response rate in cats with macroscopic ISS in small studies
- Response is usually not durable so it is unclear if it prolongs survival times in cats with gross disease
- May be more helpful in treating cats believed to have microscopic metastatic disease or cats with incomplete surgical excision when radiation therapy is not an option
Prevention
- Only vaccinate for diseases that the cat may be exposed to, or if required by law
- Vaccinate as distally as possible on limbs so they can potentially be cured of their local disease with amputation alone if ISS develops
- Standardize and record sites of injections
- Any vaccine containing rabies antigen in the right pelvic limb
- Any vaccine containing FeLV antigen in the left pelvic limb
- Feline rhinotracheitis and panleukopenia vaccines (FVRCP) in the right shoulder
- Give all vaccines as distally in the limb as possible
