Another prostate cancer study
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”.
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 »