Okay, Radiation is a carciogen. A carciogen very like all other carciogens. If I came across as trying to deny that radiation causes cancer, then I am very sorry, because the science behind the link is based on very large samples both in animal experiments and follow up studies on accidential human exposure and these data have been gone over with an astonishingly fine comb over and over again. Radiation is one of the most well understood carciogens. And one of the things which is very well understood about radiation is exactly how powerful a carciogen it is. If you get so-and-so many extra rems, then publically available statistics will tell you exactly how much this increased your lifetime risk of cancer above the baseline.
Radiation levels are also very easy to measure. - basic Geiger counters good enough to detect even very minor variations in background radiation can be bought for the price of a resturant meal, or built by anyone capable of reading basic blueprints and not soldering their own tumb onto a circut board. Scientific grade radiation measurement equipment is even more ridiculusly sensitive, and ubiquitous because it is so very useful in nearly every branch of science.  
What this means is that the level of additional radiation in northern japan is not in doubt. A conspiracy to lie about it would simply not work - the published numbers have to be correct, because anyone and everyone down to highschool students looking to score extra credits can check them.

Finally, northern japan just did not recive enough rems to do anything to the cancer rate. Which is why I objected so vorciousferly to the "uninhabitable" claim. It is simply wrong. The evacuation zone did recive doses that would statistically matter, but the only people in it at that point in time were the emergency workers.