It Didn't Prove That
According to one of the authors of a paper that showed an increase in cancer rates for two or three doses that are several times greater than the dose in the Fukushima exclusion zone:
It didn't prove that. It merely proved the safe acute dose is somewhat less than several mSv. (That's assuming something published in The Lancet was accurate and that the radiation didn't suppress other cancers.)
"This paper confirms that radiation, even in relatively low doses, does lead to risk" of certain cancers, said Alan Craft, emeritus chair at Newcastle University and an author of the paper. "There is no safe dose."