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The Dutch–Belgium randomised controlled trial of low dose CT screening (NELSON) is nearing the time it will report on the primary outcome of lung cancer mortality. The research team has published unique and influential data on the probability of malignancy in indeterminate pulmonary nodules according to their volume, growth as measured by volume doubling time and whether they are newly detected on incident screens.1 ,2 The NELSON investigators address another important factor influencing effective low radiation dose computed tomography (LDCT) screening: the maximum time interval allowable between screens before there is an unfavourable stage shift in cancers detected by screening and the rate of missed ‘interval’ cancers increases.3 In NELSON, subjects were screened at baseline, and then 1, 3 and 5.5 years from baseline. This gave intervals of 1, 2 and 2.5 years. Following an interval of 2.5 years, there were significantly more late-stage screen-detected cancers than for the 1-year interval and there were more interval cancers (5 during 1 year, 19 during 2 years and 28 during 2.5 years), with more than half appearing in the last 6 months of the 2.5-year interval (figure 3).3 The authors make the specific conclusion that a 2.5-year interval after two incident screening rounds is likely to reduce the effectiveness of screening but are more circumspect about recommending a 1-year or 2-year interval.
In the paper, table 3 shows how the number of screen-detected early-stage cancers falls as the screening interval increases, with statistical significance reached for the 2.5-year interval versus the 1-year interval. It could be argued that there is a trend across the 1, 2 and …