Getting cancer is a lot like playing the lottery (or perhaps more accurately, Russian Roulette). It requires an accumulation of mutations in two or more key genes, and these mutations occur more or less randomly throughout the genome, regardless of cause. So maybe your first hit happens from a chest X-ray, and the second hit comes from smoking. Or maybe you just get old and the replication errors pile up in blood stem cells, giving you leukemia.

The point is that it's all probablistic, and that's why some smokers live to be 100 cancer-free, and some health-nut marathoners get cancer. In general, though, these external factors can bend the odds one way or the other.