July 11, 2014

Another prostate cancer study

Today’s prostate cancer risk factor, per the Herald, is vasectomy. The press release is here; the paper isn’t open-access.

This is a much more reliable study than the one earlier in the week about cycling, and there’s reasonable case that this one is worth a press release.

In 1986, the researchers recruited about 50000 men (health professionals: mostly dentists and vets), then followed them up to see how their health changed over time.  This research involves the 43000 who hadn’t had any sort of cancer at the start of the study. As the Herald says, about a quarter of the men had a vasectomy, and there have been 6000 prostate cancer diagnoses. So there’s a reasonable sample size, and there is a good chance you would have heard about this result if no difference had been found (though probably not via the Daily Mail)

The relative increase in risk is estimated as about 10% overall and about 20% for ‘high-grade’ tumours, which is much more plausible than the five-fold increase claimed for cycling.  The researchers had information about the number of prostate cancer tests the men had had, so they can say this isn’t explained by a difference in screening — the cycling study only had total number of doctor visits in the past year. Also, the 20% difference is seen in prostate cancer deaths, not just in diagnoses, though if you only consider deaths the evidence is borderline.  Despite all this, the researchers quite rightly don’t claim the result is conclusive.

There are two things the story doesn’t say. First, if you Google the name of the lead researcher and ‘prostate cancer’, one of the top hits is another paper on prostate cancer (and coffee, protective). That is, the Health Professionals Followup Study, like its sister cohort, the Nurses Health Study, is in the business of looking for correlations between a long list of interesting exposures and potential effects. Some of what it finds will be noise, even if it appears to pass sanity checks and statistical filters. They aren’t doing anything wrong, that’s just what life is like.

Second, there were 167 lethal prostate cancers in men with vasectomies. If the excess risk of 20% is really due to vasectomy, rather than something else, that would mean about 27 cancers caused by 12000 vasectomies. Combining lethal and advanced cases, the same approach gives an estimated 38 cases from 12000 vasectomies. So, if this is causation, the risk is 2 or 3 serious prostate cancers for every 1000 vasectomies. That’s not trivial, but I think it sounds smaller than “20% raised risk”.

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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
    Nick Iversen

    Men get the snip because they don’t want any more children, which is because they have enough children already, and it’s a well known fact that children give you cancer (amongst a whole host of other diseases such as alcoholism and stress-related diseases).

    Hence the correlation between the snip and cancer.

    3 years ago

    • avatar
      Thomas Lumley

      Neat, but it would only be valid if men with vasectomies had more children than average (which I don’t think is the case) and if men with more children were more likely to get prostate cancer (which isn’t the case)

      3 years ago

  • avatar
    Nick Iversen

    Try this then: men who like children the least a) get more vasectomies (but still average the same number of children), b) spend more time away from the house cycling and c) drink more – both the diesel fumes and the alcohol give them more cancer which includes prostrate cancer. That explains both studies.

    3 years ago

    • avatar

      What about a direct physiologic effect as a mediator rather than a socio-epi sort of “behavioral confounding” explaining this trend in men who elect for vasectomy? There are significant physiologic changes to the body post-vasectomy, such as interstitial fibrosis, thickening of seminal vesicles, etc. More tissue means greater risk of (cancerous) disease. Dr Lumley is right that this was a (useful) sleuthing expedition but there is no super hard evidence for etiology yet. But it’s possible that this is a true signal.

      3 years ago

      • avatar
        Thomas Lumley

        Yes, it certainly could be real. [I have a feeling “you have been trolled, you have lost, have a nice day” may apply here, though]

        3 years ago

        • avatar
          Nick Iversen

          Don’t know what constitutes trolling but there first sentence in the Herald is “Men who undergo the snip increase their risk of suffering fatal prostate cancer” and I was pointing out that this may not be true.

          3 years ago