Evaluation of the Mechanical Stability of a Mega-Voltage Imaging System using a New Flat Panel Positioner

摘要

Mega-Voltage systems are used in radiation oncology both for external radiation delivery and patient positioningprior to treatment. A pair of portal images compared with digitally reconstructed radiographs is currently thegold standard for positioning but new developments have made possible the use of Mega-Voltage Cone Beam CTfor better 3D setup. The non-ideal imaging geometry of the treatment unit has a direct impact on both methods.It led to the use of a reticule attachment as reference for the scale and the isocenter position on the portal images.The reticule has limited precision and occasionally super-imposes anatomical information. As for Cone Beam,the image quality crucially depends on the knowledge of the scan geometry during the acquisition. The reproducibilityof the detector position at each angle will affect the image reconstruction and determine how frequentlygeometrical calibration must be performed. The objectives of this study are to measure the flex of the detectorand evaluate its reproducibility. A RID 1640 Perkin Elmer a-Si Flat Panel is installed on a Siemens Primus linearaccelerator with a positioner similar the the one used in the Oncor product. Three original methods are used toinvestigate the behavior in space and time of the imaging system. A reticule and a Plumb Bob tip are placedalong the line formed by the isocenter and the source. Their positions projected on the flat panel for differentgantry positions are used to calculate the mechanical flex. Projection matrices obtained in a geometrical ConeBeam calibration are also used to quantify the flat panel sagging. Six full sets of data were acquired over a periodof 5 months and recorded overall mechanical flexes of 1 and 3 mm for the transversal and longitudinal directionsrespectively. The absolute magnitude of the flat panel displacement varies slightly with the method used butthe discrepancy stays within the laser precision used for alignment. The small standard deviations of the flatpanel displacement (< 1 mm) suggest great stability over time and permits the clinical implementation of patientpositioning without the reticule. More experiments on the positioner with the complete set of projection matricesneed to be performed to characterize the long-term behavior of the system and to determinate how frequentlythe Cone Beam calibration needs to be done to conserve image quality. Future work will develop a daily QA protocolto detect possible collisions that would bring the Cone Beam imaging system out of geometrical calibration.