Markerless Liver Tumor Localization Using Internal Liver Volume Delineated By Four-Dimensional Cone-Beam CT

Markerless liver tumor localization has been proposed using an internal liver volume delineated by four-dimensional cone-beam CT (4D CBCT). Liver CT was performed under mid-ventilation breath hold, and transferred to a treatment planning system (TPS) to contour the gross target volume (GTV). Subsequently, liver 4D CBCT was performed and transferred to the TPS. After bone matching between the CT and the 4D CBCT, an internal liver volume was delineated on the liver CT volume as the union of liver volumes within a breathing cycle of the 4D CBCT volumes. Then, inhale liver volume was delineated on the 4D CBCT. Next, the internal target volume was defined by expanding the GTV by referring to the liver movement within the respiratory cycle of the 4D CBCT. Subsequently, all the delineated structures were transferred to the 4D CBCT unit. Immediately before treatment, 4D CBCT was performed again and the couch was repositioned so that the liver may move superiorly to the internal liver volume boundary and inferiorly to the inhale liver volume boundary during the respiratory cycle. The target localization accuracy of the proposed method was evaluated by comparing it to a published lipiodol-based technique. Both methods were applied to a single case in which lipiodol remained inside the tumor. 3D couch repositioning vectors for the two procedures were collected for 25 fraction data of the above same patient, and the differences in the vectors were calculated. The target localization deviations of the proposed method in reference to the lipiodol-based procedure were 0.7 mm ± 0.9 mm (SD) in the lateral direction, 2.0 mm ± 0.7 mm (SD) in the superior-inferior direction, and -2.1 mm ± 0.8 mm (SD) in the anterior-posterior direction. Markerless liver tumor localization is feasible by delineating the internal liver volume and the inhale liver volume using 4D CBCT.