Differences in lung injury after IMRT or proton therapy assessed by ¹⁸FDG PET imaging

Background and purpose: To compare lung injury among non-small cell lung cancer (NSCLC) patients treated with IMRT or proton therapy as revealed by ¹⁸F-FDG post-treatment uptake and to determine factors predictive for clinically symptomatic radiation pneumonitis. Material and methods: For 83 patients treated with IMRT or proton therapy, planning CT and follow up ¹⁸F-FDG PET-CT were analyzed. Post-treatment PET-CT was aligned with planning CT to establish a voxel-to-voxel correspondence between PET and planning dose images. ¹⁸F-FDG uptake as a function of radiation dose to normal lung was obtained for each patient. PET image-derived parameters as well as demographic, clinical, treatment and dosimetric patient characteristics were correlated with clinical symptoms of pneumonitis. Results: The dose distributions for the two modalities were significantly different; V5 was higher for IMRT, whereas V60 was higher for protons. The mean lung dose (MLD) was similar for the two modalities. The slope of linear ¹⁸F-FDG-uptake – dose response did not differ significantly between the two modalities. The MLD, slope, and 95th percentile of SUV were identified as three major factors associated with radiation pneumonitis. Conclusions: Despite significantly different dose distributions for IMRT and for protons, the slope of the SUV–dose linear regression line previously shown to be associated with RP did not differ between IMRT and protons. Patients who developed radiation pneumonitis had statistically significantly higher MLD and higher slope regardless of treatment modality.