If you consider the underlying mechanisms, it becomes clear. Cigarette smoke contains mutagens. For those who are genetically predisposed, it might only take a single point mutation (change of a DNA base pair) in a single oncogene to cause a tumour. So while unlikely, there is no need to suppose a threshold.

In low quantities of a toxin, up-regulation of repair mechanisms can sometimes improve outcomes (i.e. hormesis), but the risk of damage to critical genes still exists.