Two articles and an editorial in the New England Journal of Medicine (NEJM) this week look at the efficacy of screening for prostate cancer. They are available on line here, here and here.
Both studies showed that the benefits of screening are limited, though in both studies the screened group has a higher rate of cancer diagnosed than the unscreened. In the European study in each center there was “a lowering of the death rate from prostate cancer associated with screening.” This benefit was limited to those between the ages of 55 and 69.
“During a median follow-up of 9 years,the cumulative incidence of prostate cancer was 8.2% in the screening group and 4.8% in the control group. The rate ratio for death from prostate cancer in the screening group, as compared with the control group, was 0.80 (95% confidence interval [CI],0.65 to 0.98; adjusted P=0.04). The absolute risk difference was 0.71 death per 1000 men. This means that 1410 men would need to be screened and 48 additional cases of prostate cancer would need to be treated to prevent one death from prostate cancer.”
The problem with PSA testing is false positive PSA tests (positive tests in patients without cancer) and the diagnosis of non aggressive lesions that would not be clinically significant during the patient’s lifetime. Currently biopies are done on all such patients and definitiive treatment offered (in the U.S.) to almost every patient with cancer even if they appear to be of the non-aggressive variety.
In the U.S. study it was found that
“screening was associated with no reduction in prostate-cancer mortality during the first 7 years of the trial (rate ratio,1.13), with similar results through 10 years, at which time 67% of the data were complete.”
“However, the confidence intervals around these estimates are wide. The results at 7 years were consistent with a reduction in mortality of up to 25% or an increase in mortality of up to 70%; at 10 years, those rates were 17% and 50%, respectively.”
Meaning the results are not as yet statistically significant.
There are number of explanations. First, the test is not specific enough to be effective–it picks up insignificant tumors and people without evidence of malignancy. Second, the control group may have experienced an effect simply from being in the study. That is, by being in the study the patient and his doctor were more aware of prostate screening and may have introduced an artifact into the result by being more aware of the problem and testing more for indication than would have otherwise occurred. At the start of the study some PSA screening was done in both groups and some cancers detected before entry, eliminating “some cancer detectable on screening from the randomized population. Third, it was noted that there was improvement in therapy that may eliminated the benefits of screening.
It’s clear from the study that PSA screening is marginally effective procedure. Many false postives result in excessive diagnostic biopsies. Many tumors are diagnosed that would not progress to invasive cancers. But, importantly, mortality from prostate cancer has dramatically decreased since screening began.
We need a new test, one with excellent sensitivity but also greater specificity and one that would discern between tumors that would follow a more benign and malignant course. For now it appears that patients will continue to decide whether to be tested and whether to accept the risk of false positivity.
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