Figure 2. Workflow of treatment delivery.

On-board kV CT is used for the initial setup to match the three fiducial markers to the positions on the planning CT. The translational correction and the roll rotational correction can be made by the couch shift and by the adjustment of the gantry start angle while the pitch and yaw rotational error cannot be corrected. Intratreatment 2D kV radiographs are obtained six times (25, 75, 155, 205, 255, and 335 degrees) per one gantry rotation. Several parameters to control tracking SBRT delivery are shown, and if these parameters continuously exceed the threshold, we acquire another onboard kV CT to correct the motion shift.

Abbreviations: SBRT, stereotactic body radiation therapy; CT, computed tomography; kV, x-ray tube voltage