Homoculous, your bias is showing.
Rory didn't suggest that "..a bit of radiation prevents malignancy!"
He cited data which suggests that the accepted model of radiation damage ("any radiation is bad for you!") may not be accurate, and that very low levels may not be harmful.
When one considers the multiple layers of cellular protection preventing the propagation of transcription errors and the various efficient DNA repair mechanisms running this shouldn't be too surprising.
Mac