Have you a source for that? I could well believe that the cancer risk is low and obviously is is uncommon for a dose of radiation to be large, but 'comparatively large' doses of radiation are given regularly in hospital catheter labs, CT/PET scanners. And while generally these would be in the 10-20-30 mSv range, for many patients scans are repeated regularly for long time periods, rapidly reaching the 150-500mSv range discussed in the below article. Of course this is a fractionated dose and it would be much more dangerous as a single hit. I cannot find any reference to radiation dose verses cancer risk. Somewhat supporting you is this article which discusses over estimation of dose risk. Interesting discussion too: http://www.nejm.org/doi/full/10.1056/NEJMc073513

Unfortunately that doesn't correspond to any reading I've done on this topic. I'm pretty sure that chronic, sublethal exposure to radiation significantly increases your chance of various types of cancer.