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 <15months 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 20Gy biologically effective dose (α/β=3Gy) 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.