Cancer-causing radiation prevents cancer. Things that make you go “hmmm...”
Or rather, being susceptible to cancer means you won’t live long enough to reproduce in that environment, so the ones left are resistant.
https://www.medicalnewstoday.com/articles/312265#Documenting-the-fallout
It is commonly believed that the survivors of Hiroshima and Nagasaki have a high cancer burden, a significantly shortened lifespan, and children with high rates of mutations and abnormalities. After a careful examination of the data, the author found this to be an incorrect assumption.