When you say fact, you mean factoid. Hormesis is the nuclear community's homeopathy. The reason you don't find an increase in cancer from natural background is that the effects of the additional radiation get countered by the cells repair mechanisms though not perfectly as repair consumes energy needed for other processes. In other words, you need a megarat experiment to detect a small rise in cancer due to the added radiation.
Personally, I would like to a see an experiment where the background radiation got reduced by an order of magnitude. Of course natural oxidative stress from cellular metabolism also requires huge numbers of rats in the experiment to detect effects.