Quantitative measurement of changes in human lung density following irradiation

In a recent clinical study, we have shown that computed tomography (CT) can provide a good qualitative endpoint for the presence of radiation-induced pulmonary damage. However, CT data are also potentially useful as a quantitative measure of lung damage. Changes in lung density were quantified by comparing CT images taken before and after radiotherapy for 54 patients. Lung densities were assessed separately in the irradiated and nonirradiated regions using both the average regional densities and density distributions. For the 36 patients demonstrating visible post-irradiation lung damage on CT, the mean relative increase in average lung density was 0.20 +/- 0.10 in the irradiated regions. The mean change in the nonirradiated regions was 0.02 +/- 0.09 for the same group. For 18 patients without visible damage, the mean relative changes were 0.00 +/- 0.09 and -0.05 +/- 0.07 for the irradiated and nonirradiated regions, respectively. The results suggest that an increase in average lung density of 5% or greater is associated with the visible detection of pulmonary damage. The dose-response relationship based on this quantitative criterion was comparable to that derived using the qualitative endpoint.