A new method to assess pulmonary changes using <sup>18</sup>F-fluoro-2-deoxyglucose positron emission tomography for lung cancer patients following radiotherapy

<p><b>Background:</b><sup>18</sup>F-fluoro-2-deoxyglucose positron emission tomography (<sup>18</sup>F-FDG-PET) may be used for assessing radiation induced alterations in the lung. However, there is a need to further develop methodologies to improve quantification. Using computed tomography (CT), a local structure method has been shown to be superior to conventional CT-based analysis. Here, we investigate whether the local structure method based on <sup>18</sup>F-FDG-PET improves radiotherapy (RT) dose–response quantification for lung cancer patients.</p> <p><b>Material and methods:</b> Sixteen patients with lung cancer undergoing fractionated RT were examined by <sup>18</sup>F-FDG-PET/CT at three sessions (pre, mid, post) and the lung was delineated in the planning CT images. The RT dose matrix was co-registered with the PET images. For each PET image series, mean (μ) and standard deviation (<i>σ</i>) maps were calculated based on cubes in the lung (3 × 3 × 3 voxels), where the spread in pre-therapy μ and <i>σ</i> was characterized by a covariance ellipse in a sub-volume of 3 × 3 × 3 cubes. Mahalanobis distance was used to measure the distance of individual cube values to the origin of the ellipse and to further form local structure ‘<i>S</i>’ maps. The structural difference maps (Δ<i>S</i>) and mean difference maps (Δμ) were calculated by subtracting pre-therapy maps from maps at mid- and post-therapy. Corresponding maps based on CT images were also generated.</p> <p><b>Results:</b> Δ<i>S</i> identified new areas of interest in the lung compared to conventional Δμ maps. Δ<i>S</i> for PET and CT gave a significantly elevated lung signal compared to a control group during and post-RT (<i>p</i> < .05). Dose–response analyses by linear regression showed that Δ<i>S</i> between pre- and post-therapy for <sup>18</sup>F-FDG-PET was the only parameter significantly associated with local lung dose (<i>p</i> = .04).</p> <p><b>Conclusions:</b> The new method using local structures on <sup>18</sup>F-FDG-PET provides a clearer uptake dose–response compared to conventional analysis and CT-based approaches and may be valuable in future studies addressing lung toxicity.</p>