I honestly wouldn't doubt that it causes cancer in rats. Soy is known as the estrogen plant; if pregnant women consume a lot of soy, it's been known to actually give them female babies because of the overwhelming amount of estrogen that Soy contains which suppresses testosterone.
I've also read that 90% of the world's population of Soy has been genetically modified. From what I've read, genetically modified grains don't *seem* to have an effect on humans, but I wouldn't doubt that they could, and have had effects on other critters.
So yeah, I wouldn't doubt it. After all, in the wild, a rat's diet would include meat products.