(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Beam profile check is recommended in TG 40 and TG 142 as a part of Linac monthly QA. Here we present a simple but effective method to evaluate the symmetry change by taking maximum difference of off-axis ratios for all the test points within the flatten beam area as oppose to average of chosen points recommended in TG 142.
Methods: Profiles of two photon and five electron beams of a medical linac over a year were retrospectively analyzed. These measurements were done with a 2D detector array, with TG 142 recommended field sizes: 30x30 cm2 for photons and 25x25 cone for electrons. A 2cm solid water build up was used for photons and none was used for electrons. The central axis point symmetry, 0.5 cm spacing, was calculated for all points of measurements in cross plane and in plane over flattened field area, 24x24 cm2 for photons and 20x20 cm2. The very first beam profile data obtained immediately after the annual QA was used as the baseline and measurements from subsequent monthly QA were checked against the baseline with two
methods: 1) the average difference 2) the maximum difference.
Results: The maximum changes of beam symmetry in cross-plane and in-plane over a year calculated using average difference method for 6, 18 MV photon beams and 6, 9, 12, 16, 20 MeV electron beams were 0.21%, 0.31%, 0.31%, 0.27%, 0.17%, 0.20%, 0.21% respectively and they were 0.51%, 0.50% and 0.49%, 0.40%, 0.43%, 0.64%, 0.64% employing the maximum difference method.
Conclusion: The constancy of seven beams were well within TG 142 recommended tolerance value of 1%. Beam symmetry constancy check can be evaluated by taking the difference of measured and baseline points. The method is simple, efficient and it can be implemented in clinic easily.
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