[Comparison of Target Phase Positioning with Respiratory Motion between Four-dimensional CT and Four-dimensional Cone...

Cone beam computed tomography (CBCT) is the most commonly used technique for target localization in radiation therapy. Four-dimensional CBCT (4D CBCT) is valuable for localizing tumors in the lung and liver regions, where the localization accuracy is affected by respiratory motions. However, in image-guided radiation therapy for organs subject to respiratory motion, position verification is often performed using 3D cone beam CT or 2D X-ray images. While it is possible to collimate tumors at specific respiratory phases during breath-holding and to determine the tumor's motion range by taking inspiratory and expiratory breath-hold images, it remains difficult to track the tumor's trajectory at each respiratory phase. The aim of this study is to investigate the positional phases of targets that move with respiration using phantom experiments with 4D CT and 4D CBCT.

To simulate respiratory motion, we captured images of a moving phantom with a simulated tumor synchronized to simulated breathing using 4D CT and 4D CBCT. The simulated tumor was set to have respiratory cycles of 3, 4, 5, and 7.5 s, with displacements 20, 16, 10, 8, and 4 mm per breath. Under these conditions, 4D CT and 4D CBCT images were captured. Using the treatment planning system, regions of interest for the simulated tumors were set from the obtained images of each respiratory phase, identifying the tumor and setting the region as the target. Volume, positional error, and Dice coefficient of the target centroid in the corresponding phase images of 4D CT and 4D CBCT were measured with the treatment planning system.

The positional error of the target centroid between 4D CT and 4D CBCT was generally within ±1 mm. The Dice coefficient for each respiratory phase under each condition of 4D CT and 4D CBCT was generally above 0.8.

It has been suggested that 4D CBCT has the same detection ability as 4D CT for targets with respiratory movement.