3 ms·

Can someone ELI5 why false positives on a MRI are so bad? From a pure Bayesian PoV, you're better off with a noisy additional observation. At worst it doesn't

Can someone ELI5 why false positives on a MRI are so bad? From a pure Bayesian PoV, you're better off with a noisy additional observation. At worst it doesn't get much weight. At a pragmatic level, can't you say, hey here's something thats probably nothing, let's scan it again in 6 months? Why does an MRI necessarily lead to invasive follow ups? I get that ideally we'd have a crystal ball with 0 type I / type II errors but short of that, why is a noisy predictor bad?

foota 8mo ago
My understanding is it's liability, if the doctor decides not to look into it then they could be blamed for it if it turns to cancer.
maho 8mo ago
> At a pragmatic level, can't you say, hey here's something thats probably nothing, let's scan it again in 6 months If a doctor even _hints_ there might be cancer, the patient will have a terrible 6 months (with actual, measurable negative health impacts of the added stress). Also, at some uncertainy-level (say, 10% chance of cancer) the doctor _has_ to say something and has to schedule expensive followups to not risk liability, even though in 90% of the cases it is not only unnecessary, but actively harmful to the patients. When, on average, the cost of the screening + the harm done by a false positive outweighs the benefits of an early detection, you shouldn't do the screening in the first place.
IshKebab 8mo ago
I 100% agree. The UK recently recommended to not scan for prostate cancer because it sometimes detects cancers that don't need treatment: https://www.bbc.com/news/articles/cm20169gz44o.amp https://www.bbc.com/news/articles/cm20169gz44o.amp This seems super dumb to me. If the test detects cancer that doesn't need treatment, don't treat it!! It's still better to know it's there and that you need to monitor it. > Before you know it, you are on the operating table having your prostate removed – and we see examples of that all the time, Well fix that problem then. If someone puts a smoke detector above a toaster you don't just pull the battery and call it a day.
- srdjanr 8mo ago
  The question is how can you know if it needs treatment or not. I guess you either need to do a biopsy, or check if it's grown after N months (leaving patient scared and anxious during that time). Neither are great if most cases end up not needing treatment.
  - IshKebab 8mo ago
    If the test provides you zero information about whether it needs treating then it was never a useful test. Presumably it's more like "there's a X% chance this needs treatment". In which case you just set reasonable thresholds for X. E.g. if it's 5% you monitor it, 10% you do a biopsy, 70% you operate, etc. This is much more sensible than just not testing at all and letting people die from cancer. > leaving patient scared and anxious during that time This seems to be the actual motivation. We don't want to scare people with test results so we're just not going to test them. I think that should be up to the patient.
    - gjulianm 8mo ago
      > This is much more sensible than just not testing at all and letting people die from cancer. This is not what happens. You're assuming that if the cancer does not get detected by the screening then it never gets detected. What actually happens is that the test gives information that might actually be redundant and obtainable in less risky way. What the studies are showing is that waiting until there are other, more specific signs and symptoms of the prostate cancer results in the same survival rates.
      - IshKebab 8mo ago
        Interesting. Do you have a source for that?
        gjulianm 8mo ago
        See https://pubmed.ncbi.nlm.nih.gov/38926075/ https://pubmed.ncbi.nlm.nih.gov/38926075/. I was not aware of the ERSPC which came out late last year and gives better outcomes for screening, but overall the evidence is not super clear yet. There are possibly certain groups that can benefit from PSA screening more than others. Also, modern, more effective treatments might allow for later diagnosis with the same clinical results.
H8crilA 8mo ago
Because the patient is usually unable to handle such information correctly (the medical system sometimes too). And the whole-body-scan type of tests additionally pre-select for the high anxiety types.
RataNova 8mo ago
In real life, every additional data point has a cost...

foota 8mo ago

My understanding is it's liability, if the doctor decides not to look into it then they could be blamed for it if it turns to cancer.

maho 8mo ago

> At a pragmatic level, can't you say, hey here's something thats probably nothing, let's scan it again in 6 months If a doctor even _hints_ there might be cancer, the patient will have a terrible 6 months (with actual, measurable negative health impacts of the added stress). Also, at some uncertainy-level (say, 10% chance of cancer) the doctor _has_ to say something and has to schedule expensive followups to not risk liability, even though in 90% of the cases it is not only unnecessary, but actively harmful to the patients. When, on average, the cost of the screening + the harm done by a false positive outweighs the benefits of an early detection, you shouldn't do the screening in the first place.

IshKebab 8mo ago

I 100% agree. The UK recently recommended to not scan for prostate cancer because it sometimes detects cancers that don't need treatment: https://www.bbc.com/news/articles/cm20169gz44o.amp https://www.bbc.com/news/articles/cm20169gz44o.amp This seems super dumb to me. If the test detects cancer that doesn't need treatment, don't treat it!! It's still better to know it's there and that you need to monitor it. > Before you know it, you are on the operating table having your prostate removed – and we see examples of that all the time, Well fix that problem then. If someone puts a smoke detector above a toaster you don't just pull the battery and call it a day.

srdjanr 8mo ago
The question is how can you know if it needs treatment or not. I guess you either need to do a biopsy, or check if it's grown after N months (leaving patient scared and anxious during that time). Neither are great if most cases end up not needing treatment.
- IshKebab 8mo ago
  If the test provides you zero information about whether it needs treating then it was never a useful test. Presumably it's more like "there's a X% chance this needs treatment". In which case you just set reasonable thresholds for X. E.g. if it's 5% you monitor it, 10% you do a biopsy, 70% you operate, etc. This is much more sensible than just not testing at all and letting people die from cancer. > leaving patient scared and anxious during that time This seems to be the actual motivation. We don't want to scare people with test results so we're just not going to test them. I think that should be up to the patient.
  - gjulianm 8mo ago
    > This is much more sensible than just not testing at all and letting people die from cancer. This is not what happens. You're assuming that if the cancer does not get detected by the screening then it never gets detected. What actually happens is that the test gives information that might actually be redundant and obtainable in less risky way. What the studies are showing is that waiting until there are other, more specific signs and symptoms of the prostate cancer results in the same survival rates.
    - IshKebab 8mo ago
      Interesting. Do you have a source for that?
      - gjulianm 8mo ago
        See https://pubmed.ncbi.nlm.nih.gov/38926075/ https://pubmed.ncbi.nlm.nih.gov/38926075/. I was not aware of the ERSPC which came out late last year and gives better outcomes for screening, but overall the evidence is not super clear yet. There are possibly certain groups that can benefit from PSA screening more than others. Also, modern, more effective treatments might allow for later diagnosis with the same clinical results.

H8crilA 8mo ago

Because the patient is usually unable to handle such information correctly (the medical system sometimes too). And the whole-body-scan type of tests additionally pre-select for the high anxiety types.

RataNova 8mo ago

In real life, every additional data point has a cost...