[illys]

The author states: "the risk of dying from the screening exam was 1/660"

And demonstrates with: "This involves a radiation dose of about 30 milli-Sieverts. 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."

Sorry but there is a flaw here: calculation seems good but conclusion is completely wrong.

Calculation: increased risk ratio of cancer-related death for 30mS = 1.05^0.03S = 1.001465... So +0.15% = +0.0015 = around +1/660 (with less rounding +1/682)... fine!

Conclusion: this is not your risk of dying, but the increase of your risk of dying. If it was X%, the exam brings your risk at X% x 1.0015

X depends on the medicine quality in your country, your access to it, your health, your exposure to cancer-triggers (pollution, tobacco, food...), your DNA, your gender...

Let's state a depressing 1%, then the screening exam brings you to 1.0015%, or +0.0015% additional risk due to the screening exam = 0.000015 = rounded 1/67000. So your chance of dying from an exam-related cancer is absolutely not 1/660.

Please correct me if I did it wrong...

[snarkconjecture]

You did it wrong.

One Sv increases your absolute risk of fatal cancer by an added 5% or so. It doesn't multiply it by 1.05.

Quoting Wikipedia: "According to the International Commission on Radiological Protection (ICRP), one sievert results in a 5.5% probability of eventually developing fatal cancer based on the disputed linear no-threshold model of ionizing radiation exposure."

Also, where on earth did you get 1% as a "depressing" upper bound from? For lifetime risk of dying of cancer? It's over 15% in the US.

[scythe]

The real logical problem with his approach is not the relative risk. It's not the linear no threshold model.

It's the use of effective whole body dose to estimate the risk associated with a dose to part of the body. Exactly zero radiation biology organizations recommend this. Most explicitly caution against using effective whole body dose to estimate radiation risk. Effective whole body dose is only used for population-level estimates.

For example, one of the most radiosensitive (wrt cancer) organs is the thyroid. But his thyroid is not in the beam. Also the skin is exposed more than the interior on CT, which increases the risk of skin cancer. These corrections are standard, while alternatives to LNTM are not standard.

Then there is the effect of age. Most radiation related cancers are delayed by a long time, and the faster that cells are dividing, the greater the risk of DNA damage. But Scott is old, which is also why older workers were preferred for cleanup at Fukushima.

D_eff can get you within an order of magnitude I suppose, but you shouldn't express it with two significant figures — it's misleading precision. You could say 1/1000 or maybe 1/700? But you really need more detail for any kind of meaningful medical decision.

Anyway, that's my rant as someone studying for the board exam.

[tim333]

I tried googling the risk and it's all a bit inconclusive but:

>The linear no-threshold model is disputed by several health organizations, including the American Association of Physicists in Medicine and the Health Physics Society, both of which concluded that cancer risk estimation should be limited to doses greater than 50 mSv. Both organizations state that risks from doses below this threshold are too small to be detectable and may be nonexistent.

[inglor_cz]

"the risk of dying from the screening exam was 1/660"

For someone as smart as Scott Alexander, this is an astonishing mistake.

If CTs were such death machines, we would have seen a worldwide epidemics of CT-related cancers. There is no way you can cover up such a strong signal, given that people are screened all the time.

Edit: thanks for unleashing such an interesting debate. I guess the problem is in my perception. "Risk of dying from the exam" means lifetime risk, while my perception was "1 of 660 people who get the exam drops dead pretty soon afterwards".

[snarkconjecture]

It would be astonishing if it were a mistake.

And yes, it's a detectable signal. "in a large population-based cohort it was found that up to 4% of brain cancers were caused by CT scan radiation" --somewhere on Wikipedia

CT scans vary in dosage. Wiki gives ~10 Sv for an abdominal CT; I don't know where Scott got 30, but maybe the kidney screening is multiple scans or otherwise higher dose. Or he was wrong by a factor of 3, which is not a factor of 100.

CT scans aren't done frivolously, and the current rate of scans is hotly debated for exactly this reason. I'm a little surprised that kidney donation involves CT over MRI by default, but I'm not an expert.

[abainbridge]

The paper he links to [1] broadly agrees with his statement, eg "An estimated 1 in 270 women who underwent a coronary angiography CT at age 40 will develop cancer from that CT (1 in 600 men), compared with an estimated 1 in 8,100 women who had routine head CT at the same age (1 in 11, 080 men)".

1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4635397/

[lr1970]

> Conclusion: this is not your risk of dying, but the increase of your risk of dying.

No, it does not increase your risk of dying. Your risk of dying was 100% before the procedure and is 100% after it. We all die at the end with certainty. Risk of dying is only meaningful when you qualify it with a timeframe (let say next 5 years) or cause (let say getting terminal cancer).

[kelnos]

Um, yes, the entire point of that section of the article was to talk about the increase of risk of death by cancer.

[bradley13]

You're not wrong, but he's probably farther off that that. Danger from radiation doesn't scale linearly, although (to be extra safe) standards are set as if it is. In fact, there is some evidence that small doses of radiation can even be beneficial (hormesis).

[eru]

That discussion is already in the linked article.

[streakfix]

He also assumed that the risk increases linearly.

[wging]

Not blindly; there is a pretty extensive footnote (footnote 2) covering the linear-risk assumption.

[simonebrunozzi]

I think you are correct.