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To determine intrafraction prostate translation, rotation and deformation using a real-time tracking system.
4D Localization System ("Calypso
System", Calypso Medical, Seattle, WA) is an investigational patient positioning device to be used as an adjunct for radiation therapy. The Calypso System includes implanted markers (Beacon transponders) and external electronics consisting of a movable console and electromagnetic array, user interface, infrared cameras, and tracking station. The external electronics continuously localize the implanted transponders using electromagnetic signals. This study consisted of 12 patients from two institutions. Prior to treatment three transponders were implanted into the prostate gland. A treatment isocenter was selected as the centroid of the transponders on a small slice thickness CT study. In a single patient encounter, the patient was aligned initially to skin marks in the resting supine position on the treatment couch. The Calypso System guidance was then used to shift the table such that the centroid of the transponder coordinates matched the isocenter in the treatment room. Accuracy of this localization process was verified by the use of x-ray localization immediately prior to and following each Calypso System localization. Localization was performed at the beginning of the patient encounter and at the end (two alignments per patient). For the tracking sessions, the 3D positions of the three transponders were updated every 300 mSec for eight minutes. Analysis included calculations of displacement, rotations and fiducial registration error over the eight minute time period. Displacement was calculated as the change in transponder centroid relative to treatment machine isocenter in each of the three translational coordinates. Rotations were calculated relative to the centroid of the transponders. Fiducial registration error was calculated as the root mean square (RMS) error of inter-transponder distances compared to the plan after accounting for rotation and translation. This parameter represents the change in the geometry over the eight minute time period and indicates organ deformation.
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