Quantifying ventilation change due to radiation therapy using 4DCT Jacobian calculations

PURPOSE: Regional ventilation and its response to radiation dose can be estimated using four‐dimensional computed tomography (4DCT) and image registration. This study investigated the impact of radiation therapy (RT) on ventilation and the dependence of radiation‐induced ventilation change on pre‐RT ventilation derived from 4DCT. METHODS AND MATERIALS: Three 4DCT scans were acquired from each of 12 subjects: two scans before RT and one scan 3 months after RT. The 4DCT datasets were used to generate the pre‐RT and post‐RT ventilation maps by registering the inhale phase image to the exhale phase image and computing the Jacobian determinant of the resulting transformation. The ventilation change between pre‐RT and post‐RT was calculated by taking a ratio of the post‐RT Jacobian map to the pre‐RT Jacobian map. The voxel‐wise ventilation change between pre‐ and post‐RT was investigated as a function of dose and pre‐RT ventilation. RESULTS: Lung regions receiving over 20 Gy exhibited a significant decrease in function (3.3%, P < 0.01) compared to those receiving less than 20 Gy. When the voxels were stratified into high and low pre‐RT function by thresholding the Jacobian map at 10% volume expansion (Jacobian = 1.1), high‐function voxels exhibited 4.8% reduction in function for voxels receiving over 20 Gy, a significantly greater decline (P = 0.037) than the 2.4% reduction in function for low‐function voxels. Ventilation decreased linearly with dose in both high‐function and low‐function regions. High‐function regions showed a significantly larger decline in ventilation (P ≪ 0.001) as dose increased (1.4% ventilation reduction/10 Gy) compared to low‐function regions (0.3% ventilation reduction/10 Gy). With further stratification of pre‐RT ventilation, voxels exhibited increasing dose‐dependent ventilation reduction with increasing pre‐RT ventilation, with the largest pre‐RT Jacobian bin (pre‐RT Jacobian between 1.5 and 1.6) exhibiting a ventilation reduction of 4.8% per 10 Gy. CONCLUSIONS: Significant ventilation reductions were measured after radiation therapy treatments, and were dependent on the dose delivered to the tissue and the pre‐RT ventilation of the tissue. For a fixed radiation dose, lung tissue with high pre‐RT ventilation experienced larger decreases in post‐RT ventilation than lung tissue with low pre‐RT ventilation.