Forgot your password?
typodupeerror
Medicine Stats Science

Re-evaluating the Benefits of Cancer Screening 253

Posted by Soulskill
from the equations-that-are-impossible-to-balance dept.
uncleO writes "An article in the NY Times describes two studies that weigh the harm caused by cancer screenings against the benefits they provide. From the article, 'Two recent clinical trials of prostate cancer screening cast doubt on whether many lives — or any — are saved. And it said that screening often leads to what can be disabling treatments for men whose cancer otherwise would never have harmed them. A new analysis of mammography concluded that while mammograms find cancer in 138,000 women each year, as many as 120,000 to 134,000 of those women either have cancers that are already lethal or have cancers that grow so slowly they do not need to be treated. ... In recent years, researchers have found that many, if not most, cancers are indolent. They grow very slowly or stop growing altogether. Some even regress and do not need to be treated — they are harmless."
This discussion has been archived. No new comments can be posted.

Re-evaluating the Benefits of Cancer Screening

Comments Filter:
  • Re:Often wondered (Score:5, Informative)

    by interkin3tic (1469267) on Monday October 31, 2011 @03:03PM (#37898470)

    I guess the question is how do you tell the ones that need treatment from the ones that don't before it it too late to treat the ones that do

    Histology at the moment. What a trained pathologist can tell from a slide of stained cells is incredible. In the near future, genomic sequencing is what experts seem to be saying. [ted.com] You find a tumor, you get a biopsy, look at it under the microscope and also sequence the DNA of the cancer. Between what the cells look like and the DNA sequence, they'll be able to tell how likely it is to kill you.

    There are a number of well-characterized things a cancer cell must do to be really bad, and genomic sequencing will allow a good diagnosis as to what a cancer is doing exactly. If it's just that the cells are growing more than they should, but are otherwise playing by the rules (IE, unlikely to metastasize or start increasing the bloodflow to the tumor, and not in a critical location) keep an eye on it but it may not become a problem ever. If it is expressing several genes that will allow the cells to get into the bloodstream and take root elsewhere, chemotherapy now. Chances are much better that it will spread to critical areas like your lungs or brain and kill you.

  • by A beautiful mind (821714) on Monday October 31, 2011 @03:17PM (#37898702)
    No, this is not the case. The problem is that we're still far away from evidence based medicine.

    When evaluating what works and what doesn't, you have to tread very carefully. Sure, most people^Wgeeks know about double blind studies, but that's just the tip of the iceberg. The second edition of Testing Treatments [testingtreatments.org] came out recently (available as a free pdf on the website, although I bought it to support the authors) that explains the problems in an understandable language while not dumbing down the issues. The book comes with the recommendation of well known epidemiologists like Ben Goldacre, of Bad Science.net fame [badscience.net].

    To talk about the specifics of screening, check out Chapter 4. To recap the main points there, for screening to be worthwhile you have to look at several factors:
    • The condition to be screened for has to be important - either because of it's deadliness and/or because it affects a lot of people
    • There has to be a detectable early stage of the condition which to screen against
    • There exists an effective and acceptable treatment for the condition
    • There is a reliable screening test to detect the condition with

    The problem with lots of screening is that on the level of the population it can lead to more harm than good overall for a lot of different diseases, because of false positives, because of our psychological makeup that we'd prefer surgery for even harmless varieties of lumps in our bodies, etc. (see detailed examples in the book). In a lot of cases it happened that screening was introduced before the effectiveness of screening was established in a trial, then later trials showed that the screening was ineffective in reducing deaths or harm.

    The bottom line is that well designed trials should be conducted and based on the systemic review of those trials it should be decided whether to conduct screening or not, based on whether it's improving health outcomes or not. A lot of trials don't improve outcomes.

  • Re:Blood tests (Score:5, Informative)

    by Guppy (12314) on Monday October 31, 2011 @03:58PM (#37899336)

    Your link shows no evidence that "AFP is a crummy screening test". Were you hoping that nobody clicked the link, and just took your word that it was correct?

    My impression is that ColdWetDog was hoping whoever clicked the link would follow Wikipedia's explanation of how the statistics of screening tests work, and using that explanation, understand the logic of why AFP is not used as a general cancer screen by filling in the blanks themselves.

    But that's ok, maybe you didn't understand him, so let me elaborate a bit in steps. The "Specificity [wikipedia.org]" of the AFP test is the percentage of True Negatives (patients without cancer), divided by Reported Negatives (AFP tested negative). Now, the specificity of the AFP assay varies with the laboratory, cut-off criteria used, and particular cancer -- but something like 90% is reasonable for an AFP test (better for some cancers, worse for others, not applicable for many). That sounds good, right?

    Well, next step is figuring out your Positive Predictive Value [wikipedia.org]. The interesting thing about this parameter, is it varies with Prevalence. If you define your tested population as a group in which you already have reason to suspect cancer, you can get a pretty decent PPV. Now, elevated AFPs are rare in the healthy general population. Thing is -- while it might not seem that way emotionally -- statistically, cancer is also considered a rare health condition (from an epidemiological standpoint). The net result is most tumor biomarkers applied to the general population, end up with low PPVs [wikipedia.org] -- even tests with specificities of 90+% can end up with PPVs in the single digits or less.

    While I don't have a specific link for AFP, the general state of population-wide cancer biomarker screening is not good: http://www.nature.com/news/2011/110323/full/471428a.html [nature.com]

    but perhaps you are a genuine "shill" for one of the big pharmaceutical companies.

    Oh, you were just shitposting. Carry on then.

  • Re:indolent (Score:4, Informative)

    by mrxak (727974) on Monday October 31, 2011 @04:32PM (#37899834)

    I've lost loved ones to cancer as well, but if early detection doesn't help you, and false positives can really hurt you, then cancer screenings are doing more harm than good and costing society dearly.

    I've been growing more wary of early detection, and not just cancer, but all sorts of things. False positives are everywhere in medicine, more commonplace than we'd like to think. It's better to educate people on symptoms, screen only for things that don't have any symptoms (until it's too late), and generally people should live their lives normally and only see a doctor when they actually get sick. Annual check-ups are good for people who are uninformed about their health, or have questions they need answered, but what do they actually do for healthy people? Nothing. What do they do for sick people? Well, those sick people should've made a special appointment when they realized they were sick, not based on an arbitrary annual check-up schedule.

    If something hurts, is bleeding, or isn't working right, by all means, go see a doctor, ideally a specialist who knows all about it. If there's nothing wrong, though, you're more likely to become sick going to a doctor's office or hospital than if you just stayed home. Either you'll catch something from another patient who's there legitimately, or you'll become a victim of malpractice or treatment for false positives.

    While emergency medicine is based on worst case scenario, the rest of our medical system is all based on probabilities. The same three symptoms could mean you have x, y, or z, but you're treated for y because it's most likely. Only if treatment fails do we consider x or z. It's not a perfect system, but it's the best system for the most number of people, until we devise better tests to differentiate x and z from y. By all means, we should use cold hard statistics to weigh the pros and cons of screening. If the probability of harm is greater than the probability of benefit, regardless of the dangers of untreated cancer, we must advocate less screening.

The generation of random numbers is too important to be left to chance.

Working...