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In a large trial, screening yearly for lung cancer hasn't reduced mortality rates. Why is this? Isn't cancer best treatable when caught early? Is this because lung cancer is hard to treat anyway, because yearly is not frequent enough, or why?

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Not necessarily the root of what you are asking, but Diagnostic follow-up of positive screening results was determined by participants and their health care practitioners -- essentially, this leaves what to actually do with the positive result to the patient and/or the doctor. Some patients may refuse care and some doctors may not be equipped to deal with the problem or hesitate to farm out to specialists. –  jonsca May 8 '12 at 4:58
    
Is lung cancer effectively treatable? The common theme with cancer is that although lots of “treatments” exist, not all of them do actually prolonge life expectancy after diagnosis … –  Konrad Rudolph May 8 '12 at 10:55
    
This question is a bit vague. Rather than asking why screening doesn't prevent mortality, maybe you should ask yourself why screening would prevent mortality. –  Shep May 8 '12 at 12:41
    
Shep I answered this on the topic. It should prevent mortality by giving a patient a chance to start a treatment earlier, which is said to increase dramastically the odds of recovery. –  Dokkat May 8 '12 at 15:26
    
You'll get better answers if you make it clear what you're asking. I assume it's not whether cancer is easier to treat if it's caught early, or if lung cancer is hard to treat anyway, I think we both know those things are true. Do you want to know why we don't screen more? Why lung cancer is hard to treat even when we've found it? Why we typically don't catch lung cancer earlier? These are all questions that can be answered. Overall it's a good question, though. –  Shep May 8 '12 at 18:06
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There are a number of reasons, generally, why a screening test may fail to decrease cancer mortality rates:

  1. The screening test may not be very good. I know this seems like an obvious one, but its something of a problem - a screening test will only reduce mortality if it catches cases that are both treatable and wouldn't be detected in time to treat using other methods. For example, it may be that even if caught slightly later by other methods, lung cancer is equally treatable (or equally difficult to treat).
  2. Mortality from false positive tests and subsequent procedures in non-cases offsets any gain in survival among cases.
  3. Diagnosis is only one step toward treating a disease. Even if detected, if a disease is not treatable or isn't treated, knowing you have the disease doesn't change your mortality risk.

In this case it appears that a chest radiograph isn't good enough at detecting lung cancer to manifestly improve mortality outcomes. The author's of the paper don't really specify why this is - though since they're just looking at lung cancer mortality, it's likely not #2.

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