Program Information
Comparison of Primary Off-Axis Ratios for TrueBeam Varian Accelerators
A Dimofte*, K Hogue , J Li , T Zhu , University of Pennsylvania, Philadelphia, PA
Presentations
SU-I-GPD-T-426 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To compare primary off-axis ratio (POAR) and beam off-axis softening coefficient, and to obtain POAR and off-axis softening parameters for Varian TrueBeam photon beams, at the same energy.
Methods: By fitting photon beam profiles, our model provides a method to extract off-axis parameters and thus characterize primary off-axis ratio and off-axis softening effects. Photon beam profiles were measured to extrapolate POAR and μ(x)/μ(0) at 5 different depths (dmax, 5, 10, 20 and 30cm) for 6, 10 and 15MV energies from several TrueBeam linear accelerators, and are fitted by this model. The fitting algorithm is implemented by a Matlab based program.
Results: The proposed model successfully fits the measured OARs. The parameters for primary off-axis ratio and off-axis softening were successfully determined for different megavoltage photon beams. POAR can be different by up to 3% and μ(x)/μ(0) can be different by up to 2%.
Conclusion: Our results suggest that the primary off-axis ratio and beam off-axis softening effects are machine and energy dependent by up to 3%, among several TrueBeam accelerators.
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