ABSTRACT
Treatments are delivered from hundreds of beams arranged around the target. Each beam is de‚ned by a source point, called a node, a direction, and a ‚eld size. Plans with the micro-MLCs may have several segments with di‰erent MLC leaf patterns for each beam. e complete set of nodes is called the path set and can range from 23 to 133 depending on the site and patient position. e image-guided system consists of two diagnostic x-ray sources mounted in the ceiling and two amorphous silicon at-panel
detectors embedded in the oor, imaging the patient from two orthogonal views at ±45° oblique angles. Target localization during patient setup and treatment delivery is achieved by comparing the live camera images with a library of digitally reconstructed radiographs (DRRs) generated from the planning CT at 45° angles through the imaging center. Based on this comparison, the tracking software calculates the di‰erences in the three translational and three rotational directions between simulation and treatment positions (couch correction parameters). Patients are positioned on a motorized treatment table with either 5 or 6 degrees of freedom, depending on couch model. If the couch correction parameters are below the threshold sets for treatment, the robot retargets the radiation beams, without the need to stop the treatment and move the patient couch.
