Beam Adjustment with the seven729
BeamAdjust is part of the Mephysto MCC software (release 1.3 and later). The software displays real-time information about the linac beam (beam profile, symmetry, etc.). Five times a second, the display is updated with new data. The user can configure the dosimetry protocol and what kind of data is displayed. An integrating mode can be used to improve statistics.
With the LA48 linear array, BeamAdjust is used in one beam direction only (inplane or crossplane).
In March 2007, Clinac Green had to be shut down for several days (causing 26 hours of downtime) due to symmetry problems* in the transverse plane (see Fig.1). The symmetry of the beam had shown strong dependence on dose rate. This cannot be explained by a wrong calibration of the beam servos, since the servo calibration is independent on dose rate.
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| Fig.1.: Realtime transverse profile measured with the LA48. Symmetry here is 4.7% at dose rate 100MU/min due to the bad ion chamber. |
The linac ion chamber was identified as the root of the problem. After the chamber had been replaced, the beam servos had to be recalibrated for all eight energies.
Beam symmetry calibration is usually done with large field sizes (25 cm x 25 cm for electrons, at least 30 cm x 30 cm for photons). The two photon beams were calibrated in the water phantom using the LA48, because of the 2D-ARRAY's limited size. The six electron energies were calibrated with the seven29 (Fig.2).
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| Fig.2: Werner adjusts one of the Electron beams. |
When connected to the 2D-ARRAY, the software can be used for the inplane and crossplane directions simultaneously.
Fig.3 shows the main screen in the 2D-ARRAY mode, where the major axes (inplane, crossplane) and the diagonal profiles are displayed simultaneously.
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| Fig.3: BeamAdjust with the 2D-ARRAY |
Each profile can be expanded to full screen while measuring. All measurements with BeamAdjust can be saved for documentation. On March 23, 2007 the BeamAdjust software saved us a lot of time.
*Note: Although not required by local regulations, inplane and crossplane beam symmetry is checked every morning for all energies during regular morning checkout QA (PTW QuickCheck). This was the first and (as of Dec09) only incident where one of our linacs required intervention due to beam symmetry problems. This tells something about the beam steering system!