Contamination during 4 years of annual CT screening in the Danish Lung Cancer Screening Trial (DLCST)

Introduction: Contamination, defined as screening in the control arm, may dilute the statistical power of randomised screening trials. We investigated the rate of contamination in DLCST during 4 years of annual CT screening.

Methods: DLCST is a randomised trial involving 4104 healthy current and ex smokers. 2052 subjects were randomised to annual low dose chest CT scans (CT group) and 2052 were given no intervention (control group). The study is performed in collaboration with the NELSON trial (Nederlands Leuvens Screening Onderzoek) and mortality data will be pooled at the end. We defined contamination as off study CT scan received for lung cancer screening purposes. Annually during the screening period, the participants were asked whether they had received a CT scan and if so the purpose. The causes were divided into the following categories: 'Off study screening for lung cancer', 'Investigation of lung symptoms', 'Investigation of incidental finding on chest radiograph', 'Participation in other scientific trial involving chest CT', and finally 'Chest CT for other reasons'.

Results: Response rates were high in both groups: 86.1% (control group), 93.6% (CT group) p<0.001.

Control group: 199 out of 7065 answers (2.8%) indicating acquisition of a chest CT during 4 years of the trial. Three answers fulfilled the definition of contamination, while 8 received chest CT due to involvement in other scientific trials. 126 reported CT scans as part of investigation for other diseases, while 10 and 52 went through investigations for abnormal chest radiograph and lung symptoms, respectively. CT group: Significantly lower number of answers (n=134) indicated acquisition of an off study chest CT. None had received off study lung cancer screening. Three were involved in other scientific trials, while 17 and 115 went through investigation of lung symptoms and chest CT for other reasons, respectively.

Conclusion: Although the frequency of off study chest CT was significantly higher in the control group, it was relatively small in both randomisation groups, and thus contamination seems to be modest and acceptable in the DLCST. Clinical Trial no.: Clinical Trials.gov Protocol Registration System (identification no. NCT00496977).