One of the scariest types of cancer that a pet can get is a sarcoma. These are aggressive and highly invasive tumors that can grow very quickly. Cat owners are likely more aware of this cancer because of the fear of it being caused by vaccines. Within the last 20 years there have been numerous studies trying to determine why sarcomas happen after vaccines, potentially implicating certain chemicals in the vaccines. However, much of this fear is based on old data and is unfounded. When I graduated vet school they were called "vaccine-induced" sarcomas. Now they are more properly named "injection site" sarcomas.
The latest issue of the NAVC Clinician's Brief has two separate discussions on this subject. The current thinking is that certain cats are genetically predisposed to developing cancer when they have focal subcutaneous inflammation. So basically whenever anything punctures the skin a small percentage of cats have a high likelihood of the inflammation turning into a tumor. Sarcomas have been associated with a wide range of causes:
Meloxicam (an analgesic)
injectable Program (lufenuron)
and many other things. Vaccines just happen to be the most common injections we give to cats, and therefore became associated with sarcomas until further research showed other factors.
I certainly don't want to make people afraid to get their cat injected or vaccinated. This was just on my mind after reading the journal, and I wanted to bring it up to debunk the idea that vaccines are likely to cause cancer. If there was a way to screen cats for the genes that lead to sarcomas we could avoid injections in these patients. Unfortunately there is currently no such test, so we are left with playing the odds. Thankfully this is a rare situation, and the huge majority of cats have no problems whatsoever. Until new technology is developed (Star Trek hyposprays, anyone?) we can't avoid injections from time to time. I still vaccinate my cats and will readily give them any injection they need. They are also microchipped.
So don't be afraid to let your cat receive vaccines or other injections. But whenever a lump under the skin doesn't go away, be sure to have a vet check it quickly.