Background: 18F-fluoro-2-deoxyglucose positron emission tomography (18F-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 18F-FDG-PET improves radiotherapy (RT) dose-response quantification for lung cancer patients.
Material and methods: Sixteen patients with lung cancer undergoing fractionated RT were examined by 18F-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 (σ) maps were calculated based on cubes in the lung (3 × 3 × 3 voxels), where the spread in pre-therapy μ and σ 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 'S' maps. The structural difference maps (ΔS) 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.
Results: ΔS identified new areas of interest in the lung compared to conventional Δμ maps. ΔS for PET and CT gave a significantly elevated lung signal compared to a control group during and post-RT (p < .05). Dose-response analyses by linear regression showed that ΔS between pre- and post-therapy for 18F-FDG-PET was the only parameter significantly associated with local lung dose (p = .04).
Conclusions: The new method using local structures on 18F-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.