Going the other way: on the internets I read that there are 5 million CT scans in the UK every year. If there was a 1:10k rate of cancer from these we would see 500 fatal cancers a year. If there was a 1:600 risk then we would see 8300 cancers a year. There are 400k cases of cancer per year in the UK as it is. So, at the top end about 2% of cancer could be hypothesized as from CT scans based on these numbers however they were extracted kicking and screaming from the case notes.

There is an interesting twist on this though - the mortality of people who get CT scans is probably much higher than the mortality of people who don't as there is probably a reason why they are getting the scan. One reason I have seen for people to get a CT scan is that they have metastasizing cancer. If you have metastasizing cancer you are probably going to get radiotherapy. Now, radiotherapy doses are quite difficult to understand as there is a big difference in the way it gets absorbed and handled, but as a layperson I look at the numbers and think that radiotherapy doses seem much bigger than CT scan doses. But I don't even know how I would go about comparing them and controlling for them in the stats.

I personally would have to sit and think for a long time about how to sort the causal factors out in the stats around this, I think I would not be doing that on a sample of 1k people.

Develop cancer, or die of cancer? Alexander seems to be claiming the latter.