Feline injection-site sarcoma (FISS), also sometimes also referred to as Vaccine-Associated Sarcoma (VAS), is a type of malignant cancer that has been linked with the use of certain vaccines.
FISS is incredibly aggressive. If you notice a lump on your cat anywhere near a spot where they have previously received a vaccine please take them to the vet for a biopsy as quickly as possible. Do not delay in getting a tissue biopsy, because waiting even a few days can result in the tumor growing too large to safely and completely remove.
Disease Information Sites
There isn’t currently a website devoted directly to FISS, but there is a good description on Wikipedia.
Feline Cancer, Feline (Cat) Cancer Support Group, and Cats with Multiple Medical Conditions are all Facebook groups that may be helpful.
Vaccine Associated Sarcoma Support Group is an email-based support group through groups.io. This group was formerly on Yahoo. The group does not appear to be active on the groups.io platform yet, but may still have activity as an email-based group through the original Yahoo link.
Cats Health and Behavior is an email-based support group on groups.io that deals with a wide variety of topics, including cancer.