• Division of Cancer Prevention, National Cancer Institute, National Institutes of Health, Bethesda, MD, USA. pinskyp@mail.nih.gov

BACKGROUND: Recently, the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial published 7-year complete prostate cancer mortality results, which showed no benefit of screening with prostate specific antigen (PSA) and digital rectal examination (DRE). An issue of concern was the substantial level of 'contamination', or use of PSA and DRE in control arm men.

PURPOSE: To provide a detailed description of contamination in PLCO.

METHODS: Surveys inquiring about the most recent PSA and DRE use were given to a sample of control arm men throughout the screening phase of PLCO (years 0-5). A probability model was utilized to translate survey results into actual frequency counts of tests. To assess the impact of contamination, Surveillance, Epidemiology, and End

RESULTS: (SEER) incidence rates from the pre-screening era (1985-1987) as well as contemporaneous rates, were applied to PLCO person-years of observation. Results Of 38,350 control arm men, 2427 were surveyed. Pre-trial screening and college education were statistically significantly associated with increased contamination rates. The estimated mean number of screening PSAs (DREs) in the control arm was 2.7 (1.1); this compares to 5.0 (3.5) in the screened arm. 1984 and 2538 prostate cancers were observed in the control and screened arms, respectively, during the screening phase. In the absence of screening, 960 and 949 would have been expected; with contemporaneous incidence rates, 1630 and 1611 were expected.

LIMITATIONS: Due to the limitations of the surveys, in terms of both reach and scope, the exact level of PSA and DRE use in control arm men cannot be known.

CONCLUSIONS: Use of prostate screening by control arm men was substantial, but also substantially less than in screened arm men. Detailed quantitative analyses of screening use across arms are critical for understanding current and future findings from the prostate component of PLCO.