Low all-cause mortality in the volunteer-based Rotterdam section of the European randomised study of screening for prostate cancer: self-selection bias?

OBJECTIVES: The likelihood of self-selection bias in a cohort of men that entered the volunteer-based randomised prostate cancer screening trial was assessed. In addition, the accuracy of the randomisation was evaluated by comparison amongst the trial arms. SETTING: The Rotterdam section of the European Randomised Study of Screening for Prostate Cancer (ERSPC). Participants were recruited from population registries and randomised after receipt of written informed consent (49% participation rate). METHODS: Data from 37,614 men who entered the Rotterdam trial between June 1994 and July 1999 were linked to the Causes of Death Registry of Statistics Netherlands. For the assessment of self-selection bias, mortality in the trial cohort and the area of Rotterdam was compared by calculating standardised mortality ratios (SMRs, 95% confidence interval CI). Comparisons between the screening and the control arm were made by means of relative risk (RR). RESULTS: There were a total of 1661 deaths (4.4%) in the trial cohort, during an average follow-up time of 2.8 years. The observed number of deaths in the two arms combined was lower than expected (SMR 86.7%, 95% CI 81.6-92.1), which was reflected in the major cause of death categories, especially in all malignant neoplasms. No difference was observed in the all-cause mortality rate among the screening and control arm, aged 55-69 years at entry (RR 1.00, 95% CI 0.89-1.12), nor when all causes, excluding prostate cancer mortality, were considered. CONCLUSIONS: The preliminary results show that the screening and the control arm were comparable, but men recruited for the trial seemed healthier than men in the target population.