Purpose of this study was to determine the rate of contamination, defined as lung cancer screening in the control arm, of the Dutch-Belgian randomized lung cancer screening trial (NELSON) as contamination adversely affects the power of a trial. The NELSON cohort includes 15,822 high-risk current and former smokers, aged 50-75 years, equally randomized to the screen and control arm. Questionnaires were sent to a sample of 1460 male subjects of the control arm, stratified on smoking determinants. The response rate was 73.0%. The participants were asked whether they received a chest X-ray or CT scan in the last 4 years and, if so, when and for what reason it had been performed. Examinations performed after randomization because of "Precaution" or "No examination was offered by NELSON" were regarded as contamination. In the first 24 months after randomization 3.1% (2.3-3.8%) of the respondents received a lung cancer screening examination. Contamination reached a non-significant peak within the first 3 months after randomization, with a lower limit of 2.5 and an upper limit of 3.1 per 1000 person-months. This screening rate did not differ from the background rates in the last 18 months before randomization. No significant differences were observed between current and former smokers. In conclusion, the rate of contamination among male subjects of the control arm of the NELSON trial is low and is not likely to jeopardize the power of the trial.
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