The age-standardized incidence rate of prostate cancer in the male population of Quebec, aged 50 years and more, rose sharply from 1989 to 1993, whereas the age-standardized mortality rate slowly decreased from 1995 to 1999. This increase in incidence was presumably caused by the detection of preclinical cases following the introduction of PSA screening. By dividing the study population into 15 birth cohorts and into 15 regional populations, we aimed to assess whether the rise in incidence was correlated with the fall in mortality.
Our results do not show the anticipated inverse correlation between the differences in incidence rate and the differences in mortality rate. In fact, in both study populations, even if the rate differences were in the expected direction (increase in incidence and decrease in mortality), the sizes of the changes were not negatively correlated. In other words, we did not observe that a larger increase in incidence, due to PSA screening, was associated with a larger fall in mortality 6 years later.
These results need to be considered with caution. First, we relied on a single year for the increase in incidence (1993) and a single year for mortality reduction (1999). In fact, the change in incidence of any single year should be reflected downstream over several years. Moreover, we did not measure directly the degree of exposure to PSA screening. There is, however, a clear consensus among health researchers that the changes in prostate cancer incidence rates in the early 1990s are good indicators of the extent to which PSA testing was used for screening purposes.14,15
Second, because prostate cancer incidence and mortality rates are strong functions of age, the correlation performed with the birth cohorts might be distorted by age, even though we took aging into account while computing the rate differences. Nevertheless, the correlation test performed with the rate differences of the 15 regions is not limited by this, because in this case all the rates were age-standardized. Finally, it is probable that the completeness of the Quebec cancer registry with regard to this cancer decreased over the study period, because individuals with prostate cancer were being managed increasingly as outpatients and the Quebec cancer registry relies exclusively on hospital discharge sources. This underestimate of prostate cancer cases should affect all age groups and all regions equally.
Other researchers have studied the possible relation between PSA screening and the decline in the rate of prostate cancer mortality and have suggested that there is no link between the intensity of screening activities and the recent decrease in prostate cancer death rates. Etzioni and colleagues addressed the question of whether the PSA tests conducted in the late 1980s and early 1990s could provide a reason for the decline in prostate cancer mortality observed from 1992 through 1994 in the United States.11
Using simulation models, they showed that if PSA screening reduced prostate cancer mortality by 20% over 10 years (as postulated in the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial16
) and if the mean lead time (time by which diagnosis is advanced by screening) was close to 3 years, PSA screening could explain most or all of the decline in prostate cancer mortality since 1991 in the United States. However, using the hypothesis of a 5-year lead time, which better corresponds to our current knowledge about the mean lead time associated with initial PSA screening17
(the lead time for repeat screens is much longer), PSA screening could not explain the recent decline in prostate cancer mortality rate.11
In another study, Oliver and colleagues compared data from the United States with those from England and Wales with regard to age-standardized prostate cancer incidence and mortality rates.12
The authors noted that in spite of very divergent trends in incidence reflecting a different use of PSA testing, a similar reversal in secular mortality trends was observed in the United States, England and Wales between 1991 and 1997. They, thus, concluded that falling death rates in the United States did not, for the moment, support claims for the effectiveness of prostate cancer screening.
Randomized trials that address the issue of the efficacy of screening for prostate cancer are underway in Europe and in North America.18
Only one group has published preliminary results, and these suggest that PSA screening markedly reduced prostate cancer mortality up to 5 years after screening.19
However, in this report, most of the results did not come from the comparison of screened and unscreened groups as randomized, and they may reflect more bias than real efficacy.20,21
Moreover, when the groups formed by randomization were compared, prostate cancer mortality rates were identical in the 2 groups.21
Therefore, in accordance with the observational studies described here, our results do not support the hypothesis that the present decline in prostate cancer mortality is attributable to PSA screening. If PSA screening is effective in preventing or postponing death from prostate cancer, its impact at a population level has yet to be felt. Moreover, there may be other explanations for the recent decline in prostate cancer mortality, consisting primarily of changes in disease management and in hormonal treatment of advanced disease.22,23