Except that's not how it works at all, because a) risk doesn't scale linearly with exposure, b) there's a huge difference between external and internal exposure, and c) you seem to be assuming that all cancer/cancer is can be traced to radiation exposure, which is nowhere close to the truth. In fact, it's a tiny fraction, far behind things like chemical exposure and plain old random genetic mis-transcription.
Heck, even in the hardest hit areas in Japan from the Fukushima disaster, the WHO estimates only a 4% increase in the lifetime risk of hard tumor occurrence for female children (the highest risk group) with significant internal exposure to radioisotopes. And to clarify, that's a 4% increase to the rate, not an additional 4% likelihood of getting cancer.
The same study estimates the highest change would be in the rate of thyroid cancer, an increase of 70%, but even there, the change in the absolute chances lifetime occurrence is only estimated to go from like .5% to .8%. And that's a cancer that's particularly linked to internal exposure, so not particularly germane to the scenario being discussed here.