June 18, 2003

(Journal of the National Cancer Institute) -- A new study estimates that one half of the cancers detected by an annual screening program for prostate cancer among men between the ages of 55 and 67 would not have been diagnosed in the absence of screening during the patients' lifetimes, a phenomenon known as overdetection. The study appears in the June 18 issue of the Journal of the National Cancer Institute.

Screening for prostate cancer by measuring the prostate-specific antigen (PSA) can advance the time of diagnosis (known as lead-time bias) and result in overdetection. Overdetection can lead to unnecessary treatment, and such treatments have been associated with problems such as impotence and urinary incontinence.

Gerrit Draisma, Ph.D., of the Erasmus MC, University Medical Center Rotterdam in the Netherlands, and his colleagues created simulation models of prostate cancer development to estimate the lead times and overdetection rates associated with various screening programs in a hypothetical cohort of 1 million men. The programs included a single screening test at age 55, 60, 65, 70, or 75; screening every four years between the ages of 55 and 67; screening every year between the ages of 55 and 67; screening every four years between the ages of 55 and 75; and screening every year between the ages of 55 and 75.

The models were based on results from the Rotterdam section of the ongoing European Randomized Study of Screening for Prostate Cancer, which enrolled 42,376 men and in which 1,498 cases of prostate cancer were identified.

The authors estimated that a single screening test at age 55 resulted in a lead time of 12.3 years and an overdetection rate of 27%, and a single screening test at age 75 gave an estimated lead time of 6 years and an overdetection rate of 56%. Screening every four years in men ages of 55 to 67 yielded an estimated lead time of 11.2 years and an overdetection rate of 48%, and annual screening from age 55 to 67 gave an estimated lead time of 12.3 years and an overdetection rate of 50%.

The authors say that the introduction of screening would result in a 60% to 90% higher incidence of prostate cancer. They suggest that regular screening may advance the date of diagnosis by at least 10 years and that extending annual screening to age 75 would result in two cases of overdetection for every three cancers detected.

"These lead-time estimates, which are the first to be based on results of a large-scale screening trial for prostate cancer in a population-based setting, support a screening interval of more than 1 year," the authors write. "Screening for prostate cancer is likely to advance diagnosis considerably and to be associated with considerable overdetection."

They point out, however, that their estimates apply specifically to the population studied in the trial. "The intensive screening protocol, its changes over time, and the possibility of selection in the trial population should also be kept in mind when generalizing our results to other situations," they write. They say that the net balance of favorable and unfavorable effects of screening remains to be established.

In an accompanying editorial, Timothy R. Church, Ph.D., of the University of Minnesota School of Public Health, Minneapolis, says that the results are promising but notes that they should be considered "tentative and provisional". He cautions that with models like this, it is difficult to determine the degree of dependence between model input values and outcomes, and therefore whether the estimated lead-time values and overdiagnosis rates are correct.

"The potential refinement and further validation of such models could eventually lead to a clearer understanding of the role of PSA screening in the struggle to reduce or eliminate the health impact of prostate cancer," he concludes.