I am not sure either, but I can remember reading something some where about how grains and certain chemicals that are in dog foods can be a cause, I just cant seem to find the artucle that I saw it on.
When my last boxer was having seizures I did a ton of research on ingredients in dog food. What I found really disgusted me. I'm sure everyone has heard of by-products in pet foods which include stuff like eyeballs, intestines, feet, beaks, feathers, euthanized cattle, pigs, cats and dogs, etc...these things are allowed to be in pet foods ( that also includes their collars, metal ID tags and any medications the animal had in his or her system at time of death). Not very appetizing, is it?
While there is no proof that these things cause cancer I have read that artificial preservatives, like BHA, BHT, and Ethoxyquin have been found to be linked to cancers. I'm not sure if these things feed certain types of cancers, or cause them, but these are the three ingredients that I will not take a chance in feeding my pets.
Good luck with your mom's dog, I hope it is not cancer.
Like Hanna said sugar is the one thing that cancer cells feed on, so you would want to make sure there is not alot of sugar. If your questions was "did the food cause the cancer" probably not, there are things that link sugar and grains to cancer, but to my knowledge there is no hard proof of that, most of the things I have seen have been published by other dog food companies....
There are plenty of mass found that are not cancer and hopefully that is what you are dealing with... I have seen plenty of tumor/mass shrink with antibiotics...
But, I do know that keeping the immunity up is the important thing and Fish oil is a great source at around 1000 mg per 20 lbs of dog each day, this is a recommended supplement for dogs with cancer