I found this and many more in Dr. Fox's very very long article on his website:
Mutation Research/Genetic Toxicology and Environmental Mutagenesis, 2003, Vol. 539295-201. This paper from the Biology and Biotechnology Research Program, Livermore, California is entitled “Mutagenic activity and heterocyclic amine carcinogens in commercial pet foods” and the paper says that 25 varying brands of tinned cat food were tested for carcinogens in them and that out of the 25 cat foods tested, 24 of the cat foods were found to have carcinogens which could cause cancer in the cats fed these pet foods. Presumably dry pet food will also contains these carcinogens and so pet food is clearly causing cancer in pets not least since it usually contains BHT, BHA and other cancer causing chemicals such as Ethoxyquin which is still put into British pet food to give the pet food almost indefinite shelf life. Ethoxyquin has never been tested in cats to see if it is safe but has been found in dogs to cause illness, which is why American dog owners demanded that it be removed from pet food there. linkinghub.elsevier.com/retrieve/pii/S1383571803001645http://tedeboy.tripod.com/drmichaelwfox/id88.html
This is a very loooong read, but worth it, I promise!