Afaik that's highly simplified though. You have mutations all the time. Our body has means to correct such mutated cells. It's just no 100% safe system and if it gets overwhelmed it won't be able to eat up a "bigger" tumor...

Radiation always was there and always will be there. It's just about not getting exposed to too much for our body to handle. But ok - even then it's no 100% guaranteed protection (but oh well in a world where we breathe and eat toxic stuff all the time it's not use getting hysterical over radiation) and one of course might argue where the line between an accetable and a too high dose rate should be drawn...