Quantification of regional ventilation from treatment planning CT

Purpose: We describe a method of quantifying regional ventilation from the radiotherapy treatment planning computed tomography (CT) images, with the goal of developing functional images for treatment planning and optimization. Methods and Materials: A series of exhalation breath-hold (eBH-CT) and inhalation breath-hold (iBH-CT) CT images obtained using a feedback-guided breath-hold technique for radiotherapy treatment planning was selected. The eBH-CT was mapped on a voxel-by-voxel basis to the iBH-CT using a deformable image registration algorithm. By using the average CT number over a 3 mm3 region surrounding each pair of mapped voxels, the change in fraction of air per voxel (i.e., regional ventilation) was calculated. This methodology was applied to a series of 22 patients. The calculated total ventilation was compared to the change in contoured lung volumes between the exhalation and inhalation CTs (measured tidal volume). Results: A significant correlation was found between the calculated and measured tidal volumes for the left (R 0.982) and right (R = 0.985), and for both lungs combined (R = 0.985). In the resulting images, the regional ventilation was highly variable and corresponded with the spatial distribution of differences in the CT values (Hounsfield units) between the eBH-CT and the iBH-CT images. Conclusions: A method of quantifying regional ventilation from radiotherapy treatment planning CT data sets was demonstrated. The ventilation images can be used in plan optimization to minimize injury to functioning lung