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Use of DLG Tuning to Achieve Beam Matching for TrueBeam Linacs

K Sukumar*, Y Liu, E Elder, A Iwinski Sutter, H Gao, B Bradshaw Ghavidel, Emory University, Atlanta, GA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: determine the machine equivalency between two Varian TrueBeam linacs (TB1 and TB2) based on dosimetric leaf gap (DLG) parameters. Based on this information, we will be able to determine if patients can be transferred from one machine to another without having to recalculate or reoptimize the plan.


Methods: VMAT plans for various anatomical sites were selected to be recalculated for all available photon energies before being delivered on MapCHECK 2 diode array. Besides patient plans, measurements were performed using plans of different MLC leaves speeds and widths. One set of measurements compared the plans calculated using specific machine parameters of TB1 and TB2 respectively. Another set of measurements compared plans calculated using specific machine parameters of TB2 and delivered on both linacs. All measurements were analyzed using relative dose mode with 3% and 2mm parameters.


Results: gamma pass rates for patient VMAT plans were comparable for all photon energies on both machines except for 6MV. This discrepancy was attributed to the significant difference in DLG values between the machines (TB1=0.25cm and TB2=0.12cm). For the varying MLC leaves speeds and widths plans, the relative dose comparison between measurements from both machine yielded gamma pass rates in the range of 72.9% to 82.5% for plans calculated using specific machine parameters. Meanwhile, the gamma pass rates range between 82.2% and 96.6% when plans for TB2 were delivered on both machines. These results further improved to a range of 94.8% to 100% after data smoothing and shift were applied.


Conclusion: the DLG parameters for TB1 will be tuned to match that of TB2 to improve patient specific quality assurance based on gamma pass rates. Also, the gamma pass rates indicate that patients can be transferred between machines.

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