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EPID-Based Beam Matching for Linear Accelerators

B Sun

B Sun*, B Cai , S Goddu , B Morgan , D Caruthers , S Mutic , Washington University in St Louis, St Louis, MO


SU-I-GPD-T-460 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Conventional beam-matching of linacs is performed by matching depth dose as well as profiles measured in a water tank, which is labor intensive and requires beam scanning equipment. The purpose of this study is to present a novel method to match the linac beam characteristics using pixel sensitivity map (PSM) corrected EPID images and to evaluate the possibility of EPID-based beam matching.

Methods: Consistency of photon beam profiles can be used as a sensitive metric for verification of beam dosimetry including beam energies, flatness and symmetry. We have developed a novel approach to generate a two-dimensional (2D) PSM for EPIDs utilizing alternating beam and dark-field image acquisition technique. The calculated 2D PSM based on a recursive calculation algorithm was used to correct EPID pixel response. We performed a comparison of PSM-corrected open field EPID images for two standard photon energies (6 MV and 10 MV) and one flattening filter-free(6 FFF) energy between two TrueBeam linacs and an Edge linac with aS1200 imaging panels. The output factors were measured with EPID from 2x2 cm² to 40x40 cm². The difference of the beam profiles and output factors from EPID were evaluated.

Results: The three dosimetrically matched linacs with less than 1% variation in PDD, profiles and output factors measured in water scans showed an excellent agreement in the EPID profiles measurements. The 1D Gamma of the PSM-corrected profiles between the three linacs showed 100% passing rate for 6MV and 6FFF and 97% for 10 MV with 1mm/1% criteria. The maximum difference of output factors was 0.18% among all the measurements except for 2x2 cm² with 0.6% difference.

Conclusion: With preserving the beam dosimetry information, the PSM-corrected EPID images enable the possibility to match the beam data of linac dosimetry and to benchmark linac performance with the common EPID detector.

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