Monitoring of smoking-induced emphysema with CT in a lung cancer screening setting: detection of real increase in extent of emphysema

PURPOSE: To retrospectively establish the minimum increase in emphysema score (ES) required for detection of real increased extent of emphysema with 95\% confidence by using multi-detector row computed tomography (CT) in a lung cancer screening setting. MATERIALS AND METHODS: The study was a substudy of the NELSON project that was approved by the Dutch Ministry of Health and the ethics committee of each participating hospital, with patient informed consent. For this substudy, original approval and informed consent allowed use of data for future research. Among 1684 men screened with low-dose multi-detector row CT (30 mAs, 16 detector rows, 0.75-mm section thickness) between April 2004 and March 2005, only participants who underwent repeat multi-detector row CT with the same scanner after 3 months because of an indeterminate pulmonary nodule were included. Extent of emphysema was considered to remain stable in this short period. Extent of low-attenuation areas representing emphysema was computed for repeat and baseline scans as percentage of lung volume below three attenuation threshold values (-910 HU, -930 HU, -950 HU). Limits of agreement were determined with Bland-Altman approach; upper limits were used to deduce the minimum increase in ES required for detecting increased extent of emphysema with 95\% probability. Factors influencing the limits of agreement were determined. RESULTS: In total, 157 men (mean age, 60 years) were included in the study. Limits of agreement for differences in total lung volume between repeat and baseline scans were -13.4\% to +12.6\% at -910 HU, -4.7\% to +4.2\% at -930 HU, and -1.3\% to +1.1\% at -950 HU. Differences in ES showed weak to moderate correlation with variation in level of inspiration (r=0.20-0.49, P<.05). Scanner calibration could be excluded as a factor contributing to variation in ES. CONCLUSION: Increase in ES required to detect increased extent of smoking-related emphysema with 95\% probability varies between 1.1\% of total lung volume at -950 HU and 12.6\% at -910 HU for low-dose multi-detector row CT. Clinical trial registration no. ISRCTN63545820.