> the risk of dying from the screening exam was 1/660
> The usual rule of thumb is that one extra Sievert = 5% higher risk of dying from cancer, so a 30 mS dose increases death risk about one part in 660.
Well not quite. First of all, this assumes that exposure events are additive over a lifetime (rather than individual events being more damaging at higher doses). This does seem to be the case, though I'm curious if we really know.
More to the point, it's a 5% increase of chance of death from cancer. Combining 5% with 30mSv/1Sv, you get an extra 0.15% chance of dying of cancer indeed.
But the chance of dying of cancer in the U.S. is something like 17.5%, although that does increase (and then decrease) with advanced age. According to https://usafacts.org/articles/americans-causes-of-death-by-a... , something like 29% deaths in the 65-74 age group are cancer, while in the 75-84 year age group it drops again to 25%.
Regardless, this means if you figure out your lifetime chance of dying by cancer based on your current age, it's 30% at the most (in aggregate, obviously personal risk factors vary a lot from person to person).
A 0.15% increase in that is 0.045% additional chance of dying by cancer, or 1 in 2,222. Using the average mortality incidence of 17.5% its more like 1 in 3800. And for someone who's this concerned about dying of cancer and I assume limiting risk factors much more than the average citizen, perhaps it's a 1 in 5000 chance.
All in all you're much more likely to reduce your chance of death by cancer by much more than a CT will increase it, via a number of lifestyle changes: healthier eating, quitting alcohol, staying out of the sun.