Yeah cause (cancer) cells dont die instantly after getting radioactively damaged so you can/should wait a while to see the damage the radiation did and then you can hit it again.
I only know this for radiotherapy where you shoot stuff from a ray cannon.
But basically there is a lethal dosis for cells and spreading it over several days and stuff reduxes side effects but sometimes you need to be quick and therefore go for higher doses and shorter timespans. This writeup has some details if youre interested in this.
If you apply the substance into the body you have to also take elimination into consideration. Then do the maths how much you need to apply so that you reach these lethal doses and when to reapply it again.