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An EPID Based Dosimetric Verification Tool for SBRT with High Dose Rate FFF Beams

B Han

B Han1*, G Luxton1, K Bush1, E Mok1, M Lu2, L Xing1, (1) Stanford Univ School of Medicine, Stanford, CA, (2) Perkin Elmer, Waltham, MA

TU-E-BRB-7 Tuesday 2:00:00 PM - 3:50:00 PM Room: Ballroom B

Purpose: The increasing use of unflattened high dose rate and/or small sized fields in stereotactic body radiosurgery (SBRT) presents a significant challenge and calls for new tools for dosimetric measurements and quality assurance (QA). The purpose of this work is to investigate a high spatial resolution (0.2mm) and high frame rate (50Hz) amorphous silicon flat-panel electronic portal imaging device (EPID) from Perkin Elmer for SBRT.
Methods: A Monte Carlo N-Particle eXtended (MCNPX) simulation and convolution based calibration procedure has been developed to derive a voxel-based response function specific to the EPID construct and beam characteristics. Both standard photon beams and flattening filter free (FFF) beams of all energies from Varian TrueBeam STX were studied and the linearity and dose rate dependence were tested. EPID with detailed materials composition was simulated using the MCNPX to generate a scatter kernel composed of dose deposition in the EPID phosphor, and optical photon spreading and to deconvolve the EPID images to high spatial resolution photon fluence map. The fluence map was convolved with MCNPX generated kernels to the 3D dose distribution in the phantom and compared with pinpoint ion chamber and film measurements.
Results: EPID response showed excellent linearity (R2>0.9998) and dose rate dependence less than 1.8% for up to 2400MU/min. Output factors for field sizes ranging from 1x1 to 20x20cm² were measured and used to fit the optical photon glare kernel. Fluence profiles deconvolved using MCNPX scattering kernel agrees with the measurements to within 2%. Results of typical pre-treatment QA test exhibit excellent spatial resolution required for SBRT.
Conclusions: The high spatial resolution and high frame rate EPID proved to be an accurate and efficient tool for SBRT QA. Through convolution with MCNPX scattering core and comprehensive EPID calibration, accurate 3D dose maps can be generated for independent dosimetric verification of SBRT treatments.

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