September 14, 2012

Screening isn’t treatment or prevention

The US Preventive Services Taskforce has issued another recommendation against general population cancer screening, this time for ovarian cancer.  Although the USPTF guidelines don’t have regulatory force even in the US, let alone here, they are taken seriously because they are developed by people who, generally speaking, have a clue.

In some ways ovarian cancer is an obvious target for screening: it usually isn’t caught until quite late, and earlier diagnosis could theoretically be helpful in treatment.   The current screening approach in countries where screening is done is fairly sophisticated, using a combination of ultrasound imaging and a blood test for a protein produced by ovarian tumours. Even so, a large randomised trial didn’t show any benefit, and the harm from inevitable false positives was significant, since it takes surgery to follow up suspicious findings from screening.  All that can be done at the moment is to be aware of the symptoms of ovarian cancer and get them checked out. The Listener had a good article last year on the topic.

There are some cancer screening programs that are unquestionably lifesaving, eg, mammograms for breast cancer, smear tests for cervical cancer, colonoscopy for colon cancer, and (although the cost may not be justifiable) CT scans for lung cancer in smokers.  Some others, such as melanoma screening, are plausibly beneficial and unlikely to do much harm. In general, though, population screening of healthy people for uncommon diseases has a high bar to surmount: you have to find people with the disease who are treatable now but wouldn’t be treatable if you just waited.  Even more difficult, you also have to do sufficiently little collateral damage to the people (over 99.9% for ovarian cancer) who don’t currently have the disease.

 

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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

Comments

  • avatar

    Thomas, hasn’t there be some discussions on how the effectiveness of mammograms is age dependent and not recommended for women below 50 years of age? At some point the cost of intervention (false positives, extra surgeries, undesirable procedures, etc) outweighs the number of lives saved?

    12 years ago

    • avatar
      Thomas Lumley

      Yes. The benefit is less at younger ages: not only is breast cancer less common, but mammography is intrinsically less effective, so both false negatives and false positives become more important.

      For women over 50, regular mammograms reduce breast cancer deaths by about half: there are good randomized trials.

      The situtation for women 40-50 is unclear at the moment. The NZ screening system starts at 45. The US Preventive Services Taskforce controversially recommended starting at 50 rather than 40 (I don’t know if they considered 45).

      12 years ago

      • avatar
        James Stanley

        Actually, NZ used to use 50-64 as the age range from 1999 to mid-2004; and then 45 to 69 from July 2004 onwards.

        http://www.nsu.govt.nz/health-professionals/3177.aspx

        which has a link to a review article from the NZ Medical Journal (disclaimer: haven’t read it in depth!) on the right hand pane that covers benefits of screening for women under the age of 50.

        12 years ago

  • avatar
    Thomas Lumley

    That’s based on the evidence in the 2002 USPTF recommendations.

    I should point out that the USPTF updated the language in their 2009 recommendation to make it clear the issue was that the balance of risks and benefits was not clear enough to recommend screening for everyone. It’s not that they are saying no-one should be screened under 50.

    Based on the estimates in their documents, you need nearly 2000 women to have biennial mammograms from age 40 to 48, resulting in 1000 false-positive results and 60-odd unnecessary biopsies, to prevent one breast cancer death. It also looks from their estimates as if you get nearly all the benefit with about half the risk by starting at 45 rather than 40, as NZ does.

    12 years ago