(Tuesday, 7/16/2019) 10:00 AM - 10:30 AM
Room: Exhibit Hall | Forum 3
Purpose: The LA45 RaceTrack Microtron uses electromagnets to deflect the trajectory of a high energy electron pencil-beam in a scanning manner on the target to achieve a broad, flat photon beam without using a flattening filter. This study investigates the scanning parameters for LA45 to tune beam profiles during machine commissioning.
Methods: A water tank phantom was used to measure the beam profiles. The flatness and symmetry were calculated using the conventional definition as in the AAPM-TG40 report. The scan pattern file that defines parameters controlling the scanning process was modified in the machine’s service mode. First, the optimal center shift of the circular scan pattern was searched to minimize the shift between the inplane and crossplane profiles, using small scan radius (10mm). Second, a range of scan radii (120-180mm) were tested to achieve a flat beam without exceeding the maximum scanning magnet current. Third, the photon factor, which adjusts the electron striking position and angle on the target, was tuned in between 1.1 and 1.3. Finally, fine tuning to optimize the beam profiles was conducted by directly editing the coordinates and weighting of certain points in the scan pattern file.
Results: For the 45MV photon beam, a scan pattern with center shift (15mm, -19mm), radius 165mm and photon factor 1.3 was found to achieve the optimal beam profiles along both planes. To compromise the practical limit of scanning magnet current (<9A), truncation of the circular pattern in the crossplane was made to avoid triggering machine interlock, while producing negligible change to beam profiles for field sizes up to 20x20 cm².
Conclusion: The scan pattern file provides flexibility to tune and optimize the beam profiles for a scanning beam accelerator. Future work will aim to achieve a flat beam larger than 20x20 cm², when desired for other clinic applications.
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