Radiation Dose-Dependent and -Independent Pulmonary Infiltrates in Patients with High-Grade Pneumonitis After...

Background/Objectives: Analysis of the density and spatial distribution of pulmonary infiltrates of patients with high-grade (≥3) pneumonitis after radiochemotherapy and durvalumab consolidation (RT/CTx + IO) was performed in order to define dosimetric hallmarks of the development of infiltrates following this multimodality treatment. Methods: Consecutive patients treated with RT/CTx + IO for stage III NSCLC were retrospectively reviewed with respect to the occurrence of grade ≥ 3 pneumonitis. Lung infiltrates were contoured on follow-up CT scans acquired around the time of maximum pneumonitis expression. The applied dose distribution was overlaid with the follow-up CT using elastic deformation, and infiltrates were binned according to their density in density strata of 50 HU. The dose and density dependence of partial infiltrate volumes per unit lung volume was analyzed using a mixed fixed and random effect model adjusting for patient, density and dose-dependent random effects. Results: Six patients with grade ≥ 3 pneumonitis were identified from 132 patients treated with RT/CT + IO at a comprehensive cancer center. Partial volumes of lung infiltrates captured by follow-up CT with maximum pneumonitis expression ranged from 15.5 to 60.0% (median 39.8%). A significant, systematic dose-response relationship was found for partial lung infiltrate volumes per dose and density bin. A unimodal density distribution of partial lung infiltrate volumes was also found over the infiltrate density range of -1000 to 100 HU. This was determined using a mixed model that adjusted for random effects (p < 0.0001 for both effects, F-test). There was no interaction effect between systematic dose and infiltrate density dependence of the partial infiltrate volumes. The proportion of infiltrate volumes that are attributable to the systematic dose-response relation amounts to a mean of 16.6% of the total infiltrate volume per patient according to this model. Compared to patients with pneumonitis of grade ≤ 2, patients with high-risk pneumonitis had higher partial infiltrate volumes, particularly in the low-dose regions in five grade dose bins up to 20 Gy (AUC = 1.0, p < 0.0001, likelihood-ratio test). Conclusions: Dose-dependent and -independent partial lung infiltrate volumes were found in patients with high-grade pneumonitis after RT/CTx + IO. These results indicate that pneumonitis involves contributions from both radiochemotherapy-induced and immunotherapy-related mechanisms.