Well, this is what happens.
1.Evil gamma ray comes along and hits DNA
2.This damages the DNA beyond repair.
3.There is a trigger in cells that tells them 'start reproducing' and 'Stop reproducing'. The one that says 'Stop reproducing' gets damages.
4.When the cell starts to reproduce, it can't stop. We now have cancer.
As you can see, the gamma rays are focussed at the point of the tumour, so they are more likely to be destroyed than the other cells. I am not saying that those cells don't get damaged, because they do. And this starts off another cancer. If u r lucky, you would get the cancer destroyed and those cells won't be affected