My dog got cancer last year (February, 2011). A mast cell tumor, grade 2, pretty large, on the back right knee. I didn't know anything about cancer prior to that, not in dogs, at least (I know alot about human cancers). I took her in to have a biopsy, which is how I found out. These things come up rather quickly, it appears.
The first question my dog's surgeon asked, "What kind of dog food do you give her?" He recommended some brands and types. All were grain free.
She survived surgery and is cancer free. Since that day, we switched her to Taste of the Wild. We went with fish based.
To be fair, I also started giving her a multi-vitamin and (re)started her on Brewer's yeast. She's lost 15 pounds and is much more active, can jump onto the bed again, etc. She is observed to be much healthier and happier.
I think the choice of dog food plays into the health a great deal.