Between 1973 and 1991, for example, the age-adjusted death rate increased from 21 to 27 men per 100,000. However, in the early 1990s, the death rate started heading south, dropping to 24 men per 100,000 between 1992 and 1996. As the declining mortality rate dovetailed with the increase in PSA screening, the natural assumption was that the dip in deaths could be attributed to the test. To test this hypothesis, Etzioni collaborated with researchers at the National Cancer Institute to construct a computer model that would estimate the number of deaths prevented by PSA testing between 1988 and 1994, and project what the mortality would have been in the absence of such testing. Without PSA screening, for example, a man might be diagnosed with prostate cancer at age 68, but with screening doctors might catch the cancer at age 64, creating a lead time of four years.
Lead time is an important indicator of a cancer's aggressiveness.
Assuming that PSA testing significantly improves life expectancy, Etzioni and colleagues estimate that for PSA testing to wholly explain the observed mortality decline, the average lead time in the population would have to be quite short - three years or less. Prostate cancer, however, typically is associated with a longer lead time, as the disease often goes undetected for many years prior to diagnosis. While two previous studies suggested an average lead time ranging from three to five years, the results were based on subgroups of men who tend to have shorter lead times than the population as a whole, Etzioni says. Even if PSA testing improves life expectancy in a clinically meaningful way, we conclude that it is unlikely that the declines in mortality observed from 1992 through 1994 are solely due to PSA testing. It is likely that some other factors are playing a role in these declines, such as treatment innovations for advanced disease.