[quote=Darcs]
Controlled exposure to radiation over time is a surefire way to ghoulify yourself!
[/quote]

At the same time the game has shown instances where a person may turn into a ghoul with an instant blast of radiation. And even a sane one. But given no attempt at sciencing ghouls has been done (and the game's plot insists on keeping them an enigma for the most part) it's a pretty much a solid roll of the dice combined with the careful balance of the subject's mind on the knife's edge between sanity and feral.