1. Many COPD patients will have had a previous chest CT examination. This has become common in medical practice, especially in patients with chronic lung diseases such as COPD patients. Including such patients would introduce huge bias in the study by artificially focusing on rapidly growing cancers. Redoing a CT in a screening intend in a patient who recently underwent this investigation would also unnecessarily enhance irradiation in these patients. It should be note that in the NLST trial "previous chest CT (≤ 18 months)" was a key exclusion criterion [NLST Radiology 2011].