Target volume and motion position evaluation of four-dimensional cone-beam CT: comparison with 4D-CT using dynamic thorax phantom

Purpose: This study evaluated the target volume (TV) definition and motion position accuracies of 4D cone-beam computed tomography (CBCT) through comparison with 4D-CT, using a dynamic thorax phantom. Methods: A dynamic thorax phantom was used to acquire 4D images with respiration motion. Targets with 20-mm diameters, motion periods of 3, 4, and 6 s, and 1D and 3D motion patterns corresponding to sin and cos6 curves were used. The TV definition was contoured on all 10 phase images via threshold segmentation on all 4D datasets. The TV union was defined as the internal target volume (ITV). The TV and ITV were compared to the theoretical TVR and ITVR. For motion position evaluation, the TV centroid positions were measured and the root mean square errors (RMSEs) of these measured positions calculated relative to the phantom setup positions. In addition, the breathing patterns of patients with lung tumor were evaluated. Results: The maximum differences in TV and ITV for 4D-CT were 0.66 cm3 and −14.03% for a cos6 motion pattern with a 3-s period, respectively. In contrast, the maximum average differences in TV and ITV for 4D-CBCT were 0.43 cm3 and −6.48%, respectively, for all motion patterns and periods. The 4D-CT and 4D-CBCT RMSEs were within 2.5 mm for most motion patterns. For patient breathing patterns, the average differences in TV, ITV, and positions between 4D-CT and 4D-CBCT were 0.18 cm3, 0.23 cm3, and 1.1 mm, respectively. Conclusions: Although 4D-CT and 4D-CBCT have differences in image acquisition and voxel size, the TV and motion position accuracies for 4D-CBCT are consistent with those of 4D-CT.