An injection site sarcoma is a tumor of the connective tissues in the cat. The most common cell type affected is the fibroblast, giving the tumor the name fibrosarcoma. These tumors are often located between the shoulder blades, in the hip region, and in the back legs. Here is a photo of one: A cause and effect relationship has been established for certain vaccines, including rabies and feline leukemia. Fibrosarcoma can develop weeks, months, or even years after a vaccination. It is likely there is a genetic predisposition to the development of these tumors in certain cats, however, the exact genetic problem has not yet been identified. A surgical biopsy is often necessary to make a definitive diagnosis of injection site fibrosarcoma. Once diagnosed surgery is also the usual form of treatment. Surgical removal needs to be wide and deep. These tumors grow by sending finger-like projections of tumor cells among the tissues. Injection site sarcomas in cats are very difficult to treat.  Cats with tumors on the legs, where an amputation can be performed, appear to do better than cats with tumor on the trunk of the body.  Treatment combining surgery, chemotherapy and radiation therapy significantly increases the tumor free time to 18-24 months. Most cats will never develop a vaccine associated sarcoma. Vaccines are important because they protect cats against serious diseases. The best advice is to vaccinate discriminately and make educated decisions about which diseases your cat should be vaccinated for.