An injection site sarcoma is a tumor believed to be induced by an injection – most often a vaccination. These sarcomas are very rare but may occur in cats as a consequence of an overzealous inflammatory or immune system reaction to the vaccine.

A sarcoma is a malignant tumor composed of cells derived from connective tissue. These tumors often develop quickly and can spread (metastasize) to distant locations in the body. These tumors often are not responsive to treatment and result in serious illness and ultimately death of the animal. Recurrence of such tumors is common after surgical removal.

Injection site sarcomas were first recognized in the late 1980s when some changes occurred in the vaccine manufacturing process. At that time, manufacturers changed from production of modified live virus vaccines to killed-virus products as directed by the United States Department of Agriculture (USDA). This change in manufacturing process resulted in the inclusion of aluminum into vaccines. It is this aluminum component of the vaccines that is suspected to be associated with development of post-vaccinal sarcomas. The feline leukemia virus and rabies vaccines are most frequently suspected in pets that develop post-vaccinal sarcomas.

The actual incidence of injection site sarcomas is not known with certainty. Some investigators estimate that post-vaccinal sarcomas occur in as many as 1 of every 1,000 to as few as 1 in every 10,000 cats vaccinated. Injection site sarcomas are recognized only in cats.

Despite the localized appearance of these tumors, microscopic branches of the tumor extend like fingers into the surrounding healthy tissue. During surgery to remove the tumor, these microscopic branches can remain and contribute to re-growth of the tumor. According to one study, as many as 62 percent of post-vaccinal sarcomas recur within 6 months after surgical removal.

For more information, read the article Injection Site Sarcoma.