Measuring mortality reductions in cancer screening trials.

Randomized trials involving large numbers of people and long follow-up have helped measure the mortality reductions achievable by screening for cancer. However, in many of these trials, the reported reductions have been modest. Part of the reason is the inappropriate way the reductions have been calculated. Analyses have largely ignored the fact that there is a time window in the first several years after screening begins in which there cannot be a sizable mortality reduction, followed by one in which the reductions become evident, and-unless screening is continued-a third window in which mortality rates in the screened group revert to those in the unscreened group. This review uses time-specific mortality ratios to address the timing and extent of the reductions achieved in trials of screening for prostate, breast, and colorectal cancer. The author finds that the mortality reductions reported in the literature have substantially underestimated what might be accomplished with continued screening. The natural history of the disease, the frequency of screening, and the duration of follow-up determine the time patterns in the reductions observed in trials. Without appropriate analyses, results from cancer screening trials will be distorted.