The tumor results came back as an aggressive soft tissue sarcoma. When we were shaving Flower for the surgery, we noticed a scar on the right shoulder. Oftentimes these sarcomas are associated with some type of trauma or injection site reaction. Flower's scar was likely from when she was a kitten, before we knew her, as we were not aware of any wound which would result in a scar like that in the 15 years we have had her.
Cats are somewhat unique in their handling of tissue reactions, and are the only documented species that develops injection site sarcomas. This has been a concern in recent years, and vaccine manufacturers have moved away from irritating adjuvants in feline vaccines to try and reduce the incidence of these tumors. We give vaccines in different locations on cats to try to identify which vaccines may cause problems or reactions, and also to make surgical resection easier if it is necessary. So when you bring your cat in for shots, this is the reasoning behind our choice of injection sites. Your cat's risk of developing an injection site sarcoma is much lower than their risk of catching one of the diseases we vaccinate for, so don't be alarmed and think you should not have your cat vaccinated. However, we do not advocate giving shots your cat does not need, and assess risk individually for each patient. Also, any type of penetrating wound can cause the same type of problem, as evidenced by Flower and her scar.