Impact of dose on lung ventilation change calculated from 4D-CT using deformable image registration in lung cancer patients treated with SBRT

Objectives Ventilation derived from four-dimensional computerized tomography (4D-CT) using deformable image registration (DIR) has been found to correlate with xenonenhanced CT ventilation. How radiation dose affects ventilation after stereotactic body radiotherapy (SBRT) is not well understood. In this paper, 4D-CT-derived lung ventilation maps and 3D radiation dose distributions were investigated. Methods Diffeomorphic morphons were used as the DIR tool. A total of 31 SBRT lung cancer patients' tidal volume normalized ventilation and planning dose datawere retrospectively analyzed. All patients had two sets of 4D-CT, one at pretreatment and one post-treatment from which ventilation distributions were calculated. The two ventilation data sets per patient were aligned using DIR. Radiation dose distributions were resampled to match the resolution of CT images. Aventilation change and dose values were thus associated to each voxel of the CT images. A function (ventilation change)dose-volume surface was generated for each case. Results Average ventilation was found degraded in higher than 20 Gy dose regions for 25 out of the 31 cases. The ventilation degradation was statistically significant for the dose regions of greater than 20 Gy outside the tumor volume compared to the lower dose regions (p< 0.05). Conclusions This suggests that 4D-CT-derived ventilation maps can be used in treatment planning to spare functional lung volumes.