As for Radiation, there is no such thing as a threshold dose of radiation. That is, a dose where if you remain under it, you are safe, and if you go over it, you get cancer.
There is no guarantee a person will never get cancer, as the causes are multiple.
I was trained in nuclear physics as a Nuclear Medicine technologist. Granted, that was 40 some odd years ago, but not much has changed since then.
It is an accepted fact with Radiation that there is no threshold dose for cancer
I think it is because the variability in human biology. One person can be quite resistant to radiation, while another person is not.
In radiation exposure, they do have what is called the LD-50/30 dose where when this threshold is reached, 50% of those people will die within 30 days.