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Comment by hadmatter | Hacker News Reader

Funnily enough, accuracy is a somewhat inaccurate term when it comes to medical testing. What we are interested in are sensitivity, specificity [0] and prevalence of the disease in the population being tested. With those three we can get positive prediction value (PPV) and negative prediction value (NPV) [1]. But PPV and NPV always depend on prevalence! That's why we don't want to test healthy people for all kinds of exotic diseases.

[0] https://en.wikipedia.org/wiki/Sensitivity_and_specificity [1] https://en.wikipedia.org/wiki/Positive_and_negative_predicti...

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However, despite the high false positive rate, breast cancer screening is a net beneficial procedure (though not without controversy) in that when applied in a population, long term outcomes like mortality improve.

(edit yes of course cancer is not beneficial thank you)

> However, despite the high false positive rate, breast cancer screening is a net beneficial procedure (though not without controversy) in that when applied in a population, long term outcomes like mortality improve.

I've read that's also a statistically tricky statement. IIRC, they measure survival time from when the cancer was detected. So early detection by definition improves the survival stats even when there's no meaningful treatment.

If I've read about this, I'm sure medical practitioners are well aware of it, but even so, I don't think it's easy to correct for it. How do you determine when a late-detected cancer actually started?

You don't, you just measure long term outcomes like mortality in a treatment and control arm. Potentially you have to use a synthetic control.

I am most familiar with NLST which actually was a long term randomized trial.

Thanks! Paraphrasing to check my understanding: you don't use the stat I mentioned, instead measuring whether the patients ultimately die from cancer or something else. You use a control, so after like detection method some patients get treated and some don't. But not treating may be unethical, so you use a "synthetic control", which I had to look up. Basically an invented group of not-treated patients based on statistics from other populations. [0] It sounds interesting but tricky to get right. "A healthy degree of scepticism on the use of synthetic controls is thus expected from the scientific community."

[edit: or, looking at the NLST you mentioned [1], maybe "treatment" doesn't always mean what I think of as treatment. They are actually comparing two different detection methods, and they aren't using a synthetic control.]

Do they ever use the "survival time after detection" stat I mentioned anymore, or has it been (/ should it be) abandoned?

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7218288/

[1] https://www.cancer.gov/types/lung/research/nlst

Nice follow up!

Yes, NLST was actually a real randomized control trial for lung cancer screening, done before it was known whether or not it was helpful. The trial eventually stopped once they had enough outcome based evidence.

The statistic you mentioned is not used in assessing the effectiveness of screening. It does closely relate to what is though to be the causal mechanism that leads to effectiveness.

Yep, but so do anxiety disorders associated to a “possible cancer”.

Living hapyy is more important than living for lots of people.

You can still try to estimate this using something called QALYs which are life years adjusted for quality of life.

A breast cancer scare, is certainly harmful, but it's important to remember that it is going to be a short episode as it becomes clear that the tumor is not growing. So you have maybe a few months of scariness.

On the other hand cancer is brutal, and death is permanent. Years of life cut short. The years leading up that are extremely sad. The impacts of death extend beyond the person to their family and friends.

When people try their best to measure these things on balance, breast cancer screening is overall a good intervention.

Just to expand/add nuance to your comment about controversy:

It's pretty uncontroversial that cancer screening for women above 50 years old or women with high risk (e.g. familial or genetic evidence or risk, history of cancer/treatment) is a net beneficial procedure.

For asymptomatic, low-risk women aged 40-49, the recommendations are a bit more mixed. While there is some evidence that overall mortality does improve, there are also considerable harms (physical, emotional, financial) associated with false positives and "overdiagnosis" (that is, identifying and treating low-grade non-invasive lesions that are unlikely to progress to breast cancer).

Checking the Wikipedia article, for every woman saved there is 10 women who will get unnecessary surgery and 200 that will get to go through extended testing.

It would be interesting to know the mortality rate of unnecessary breast surgery. I would hope the number is less than 10%, but I suspect it is not an insignificant number of people. Net beneficial, but definitively controversial.

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