The Development of Volumetric Quantitative Evaluation Software for Assessing Respiratory-Induced Target Motion

Purpose We developed a volumetric quantitative evaluation software called vector volume histogram (VVH) to evaluate respiratory-induced organ motion using deformable image registration (DIR). Methods The B-spline-based DIR algorithm was used to compute the deformation vector field (DVF), which included the DVF_LR (left-right), DVF_AP (anterior-posterior), and DVF_CC (craniocaudal). The VVH software was written as a plug-in using Python, thus allowing anyone to easily modify the code. A shifted target within the moving phantom was used to evaluate the performance of the VVH software. The 2 cm diameter target was systematically shifted by 5, 10, 15, and 20 mm in the CC direction. To evaluate respiration-induced target motion, the VVH method was applied during the inhalation and exhalation phases of 4D CT scans in a patient with lung cancer. Length at 5% volume (L_5%) and length at 50% volume (L_50%​​​​​) were calculated to evaluate the target motion. Results In the phantom study, the VVH software accurately measured target displacements with L_5% and L_50% values of 5.4 mm and 4.8 mm, 10.4 mm and 9.8 mm, 14.9 mm, and 14.6 mm, and 19.9 mm and 19.6 mm for 5, 10, 15 and 20 mm displacements, respectively. For the lung cancer patient study, the VVH method showed target motion with L_5% and L_50% values of 1.9 mm and 1.8 mm in LR, 1.9 mm and 0.9 mm in AP, 18.8 mm and 15.8 mm in CC, and 18.9 mm and 15.8 mm in 3D vector. The centroid method measured respiratory tumor motion between the inhalation and exhalation phases as 0.5 mm, 0.7 mm, 13.5 mm, and 13.5 mm in the LR, AP, and CC directions and in the 3D vector. Conclusions The VVH software provided a volumetric quantitative assessment of respiratory-induced target motion and may provide strategic decisions for clinical use at the time of treatment planning.