Program Information
Use of Electronic Portal Imaging Device (EPID) for Quality Assurance (QA) of Electron Beams On Varian TrueBeam System
B Cai*, S Yaddanapudi , B Sun , H Li , C Noel , S Mutic , S Goddu , Washington University in St. Louis, St. Louis, MO
Presentations
MO-B-BRD-2 (Monday, March 9, 2015) 10:00 AM - 12:00 PM Room: Ballroom D
Purpose:
EPID has not been utilized to QA electron beam parameters on Varian TrueBeam LINAC due to unavailable imager calibrations and disabled dosimetry imaging acquisition mode for electron beams. This study aims to provide solutions to implement EPID-based QA for electron beams on Varian TrueBeam system.
Methods:
Ad-hoc mode electron beam images were acquired in developer mode with XML code. Large source-to-imager distance flood field (LSID-FF) was proposed and used for calibration. Over a two-month period, images were taken with a proposed diagonal-wedge phantom for all electron energies using 20 by 20 cm2 applicator. Beam parameters including output, field size, symmetry, flatness, uniformity and percent depth of ionization (PDI) curve were analyzed and compared with baseline images for constancy check. To test the sensitivity of EPID in catching energy change, Bending Magnet Current (BMI) was detuned to allow energy change corresponding to ±1.5 mm change in R50 of PDD. EPID images were then acquired using same method and compared against baseline on multiple machines.
Results:
LSID-FF calibration method is appropriate for constancy check. After correction, beam profile extracted from EPID is comparable to Sun Nuclear IC-profiler results. The two-month EPID data illustrated same trend with SunNuclear Daily QA device. 0.3 changes in uniformity and 2-unit shift in PDI curve were observed when energy changes exceed TG142 tolerance.
Conclusion:
The experimental results clearly demonstrated that electron beam parameters can be reliably measured with EPID for constancy check with proposed imaging acquisition and calibration method and phantom. Using uniformity and PDI shift as metrics, EPID measurement is sensitive enough to catch energy change according to TG142 guideline.
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