Program Information
An EPID-Based Method to Determine Mechanical Deformations in a Linear Accelerator
D Gourdeau*, L Gingras , F Beaulieu , G Leclerc , L Archambault , CHUQ Pavillon Hotel-Dieu de Quebec, Quebec, QC
Presentations
SU-J-CAMPUS-TT-3 (Sunday, July 30, 2017) 4:00 PM - 5:00 PM Room: Therapy ePoster Theater
Purpose: To develop a new QA method to measure simultaneously every mechanical deformations of a linac’s gantry and portal imager, including sags and tilts, radiation source displacement and isocenter coincidence over the course of a full gantry rotation. A total of 14 degrees of freedom are characterized.
Methods: EPID images of a widely available commercial phantom with a custom-made head-mount accessory were acquired at regular gantry angles. First, the EPID images were compared with images generated by simulation of the linac geometry, using ray-tracing from the source. Then, to minimize the differences between the EPID and simulated images, a genetic algorithm coupled with a gradient-descent optimizer was used to simultaneously adjust every degree of freedom. Phantom mispositioning and gantry angular offset were calculated and extracted from the results. Absolute uncertainty of every mechanical deformation was calculated for different levels of simulated noise. Radiation isocenter position and radius were determined by tracing the beam central axis from the source position to the center of collimation.
Results: All 14 parameters are simultaneously optimized. Average distance between the center of the measured and simulated ball bearings on the imager was 0.05 mm. A complete map of the imager and gantry mechanical deformations and source position as a function of gantry angle was produced, showing sub-millimeter translations and rotations smaller than 1 degree along every axis. Average absolute uncertainty (1 SD) was 0.22 mm or degree. The mechanical isocenter position was determined with more than 0.1 mm precision.
Conclusion: This work presents a new method for the mechanical QA of linacs, which is both more complete and more precise than currently available tests. By combining different tests into one, it saves time and allows for the gantry and imager tilts measurements. Deformations are measured with sub-millimeter accuracy and are shown to be reproducible.
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