I remember my own natural instincts about cancer screening before a friend in public health set me straight about false positives and other possible harms. I figured the more mammograms I got, the better, right? Same with tests for prostate specific antigen, or PSA, in men, no? The test can save your life. What could be bad?
A lot, of course, including potentially terrible complications from prostate surgery. And earlier this week, a federal panel issued final recommendations against routine prostate screening for healthy men. But the panel's calculations that the tests do more harm than good have failed to convince many who reason as I used to, that catching cancer early must save lives.
In a paper that couldn't be timelier, this month the journal Psychological Science publishes an analysis of the "uproar" over prostate screenings last October, when the panel issued its initial recommendations leaning in the same anti-PSA direction.
The Psychological Science paper identifies four psychological factors that "can help explain the furor that followed the release of the task force’s report." They are:
• The persuasive power of anecdotal (as opposed to statistical) evidence
• The influence of personal experience
• The improper evaluation of data
• The influence of low base rates on the efficacy of screening tests.
The authors, from Ohio State University and The Max Planck Institute for Human Development in Berlin, suggest that the reaction might be different if the panel's calculations were better explained: "Augmenting statistics with fact boxes or pictographs might help such committees communicate more effectively with the public and with the U.S. Congress."
I'm all for better explanations. I also can't help thinking that these psychological factors cannot apply to the experts whose opposition to the anti-PSA recommendations is quoted in this NPR story and in this previous CommonHealth post.
Putting potential arguments aside, the paper offers a wonderfully vivid explanation of public attitudes by presenting the thought experiment of a thousand older men in an auditorium:
Consider two auditoriums, each of which contains 1,000 men age 50 or older. Auditorium “Screened” contains 1,000 men who have had a PSA screening test. Auditorium “Not Screened” contains 1,000 men who have not had such a test. About 8 men from each auditorium will die from prostate cancer in the next 10 years. A very important conclusion to be drawn from these numbers is that screening does not decrease prostate-cancer mortality. How can this be, given that so many men claim to have been saved by a PSA test?
Let us take a look at three possible subgroups of men in Auditorium Screened. These three subgroups have in common that they received a positive PSA test result. The first group consists of those men whose positive PSA test actually did detect a progressive prostate cancer and who are still alive because their cancer was detected early and subsequently treated successfully. This group’s members rightfully believe that they were saved by a PSA test. However, given that the USPSTF [United States Preventive Services Task Force] report and some of the meta-analyses cited earlier raise doubt that PSA screening does in fact reduce prostate-cancer mortality (not to speak of overall mortality), it is unclear whether there actually are such men in the auditorium.
Even in light of the more favorable trials, such as the European Randomized Study of Screening for Prostate Cancer (Schröder et al., 2009) and its recent update (Schröder et al., 2012), it seems safe to assume that PSA screening saved at most 1 man in the auditorium from dying from prostate cancer; and in light of the less favorable evidence (Andriole et al., 2009; Djulbegovic et al., 2010; Ilic et al., 2011; Sandblom et al., 2011), quite possibly none have been saved.
The second group in Auditorium Screened consists of about 20 men who were unnecessarily diagnosed and treated, because their cancers would have never caused them harm had the cancers not been detected by screening (Djulbegovic et al., 2010). Quite a few of these men will have serious side effects, such as impotence or incontinence, as a consequence of the treatment. About a quarter of the men who have a radical prostatectomy will have such symptoms (USPSTF, 2011). Elevated risks of impotence and bowel dysfunction follow radiation treatment, too. Thus, in this subgroup of 20 men, there will be about 5 who will have to live with such consequences, which they would not have experienced had they not undergone treatment.
Moreover, these men faced a small risk of dying from the surgery: About 5 out of every 1,000 men (which equals 0.1 in this subgroup of 20 men) will die within 30 days of undergoing a radical prostatectomy. Yet these 20 men do not know that the screening did not improve their eventual outcome, so they mistakenly believe that they were saved by the PSA test. Even those with serious side effects will probably believe that the side effects are worth it, even though the screening, with subsequent treatment, only caused them harm with no benefit. Thus, they are angry about the task force’s recommendation to forgo the PSA test. Their families also think that the prostate cancer is the reason why they had to undergo treatment, and do not know that the treatments were actually unnecessary and only harmful. These families might be very unhappy with the PSA screening test if they did know.
Auditorium Screened also contains a third group of about 180 men who received a false-positive test result and had an unnecessary biopsy. Even if a man has only a biopsy with nofurther medical treatment, there is the risk of harm. Seven percent of the men who have prostate biopsies (i.e., 12.6 of these 180) have to be hospitalized because of infections and other complications (Loeb, Carter, Berndt, Ricker, & Schaeffer, 2011). These 180 men will probably not think that they have been saved, but will be relieved that their test result was only a false alarm. Probably only few of them, if any, will blame the test, however, for having produced a false alarm in the first place.
So, in Auditorium Screened, there are a lot of men who think that the PSA screening provided more benefit than harm, when in fact the opposite is true. We know that it is true because (a) the number of prostate-cancer deaths is the same in the two auditoriums, but (b) Auditorium Screened contains many men with serious side effects that could have been avoided had the men not been screened with a test that has a high false-positive rate.
We cannot blame the men in Auditorium Screened for their beliefs that they are healthier than the men in Auditorium Not Screened and that death from prostate cancer has reduced their numbers to a lesser degree. In the real world, there are no such adjacent auditoriums whose proximity allows an easy comparison of the health status of the two groups. Of course, epidemiological studies allow for such comparisons. When the men in Auditorium Screened rely on their personal experience, they are not aware of the counterfactuals. They do not know about the fate of the men who were not screened or of the men who made different treatment choices than they did. An individual man might only know that he was screened, had a biopsy, was treated, and now does not have prostate cancer. He thinks the PSA test saved his life, and he places an unkind comment about the task force members’ professional compe- tence on USPSTF’s Web site.
Readers? Convincing? Unfortunately, the full paper is behind a paywall, but the abstract is here.
This program aired on May 23, 2012. The audio for this program is not available.