Pulmonary function after lung tumor stereotactic ablative radiotherapy depends on regional ventilation within irradiated lung

Purpose To determine if regional ventilation within irradiated lung volume predicts change in pulmonary function test (PFT) measurements after stereotactic ablative radiotherapy (SABR) of lung tumors. Methods We retrospectively identified 27 patients treated from 2007 to 2014 at our institution who received: (1) SABR without prior thoracic radiation; (2) pre-treatment 4-dimensional computed tomography (4-D CT) imaging; (3) pre- and post-SABR PFTs <15 months from treatment. We defined the ventilation ratio (VR_20BED3) as the quotient of mean ventilation (mean Jacobian-based per-voxel volume change on deformably registered inhale/exhale 4-D CT phases) within the 20 Gy biologically effective dose (α/β = 3 Gy) isodose volume and that of the total lung volume (TLV). Results Most patients had moderate to very severe COPD by GOLD criteria (n = 19, 70.1%). Higher VR_20BED3 significantly predicted worse change in Forced Expiratory Volume/s normalized by baseline value (ΔFEV₁/FEV_1pre, p = 0.04); n = 7 had VR_20BED3 > 1 (high regional ventilation) and worse ΔFEV₁/FEV_1pre (median = −0.16, range = −0.230 to −0.20). Five had VR_20BED3 < 1 (low regional ventilation) and improved ΔFEV₁/FEV_1pre (median = 0.13, range = 0.07 to 0.20). In a multivariable linear model, increasing VR_20BED3 and time to post-SABR PFT predicted decreasing ΔFEV₁/FEV_1pre (R² = 0.25, p = 0.03). Conclusions After SABR to high versus low functioning lung regions, we found worsened or improved global pulmonary function, respectively. If pre-SABR VR_20BED3 is validated as a predictor of eventual post-SABR PFT in larger studies, it may be used for individualized treatment planning to preserve or even improve pulmonary function after SABR.