Program Information
Determining the Sensitivity of Beam Profile Parameters for Detecting Energy Changes in Flattening Filter-Free Beams
K Mooney*, S Yaddanapudi , S Mutic , S Goddu , Washington University, St. Louis, MO
Presentations
SU-E-T-249 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To identify the beam profile parameters that can be used to detect energy changes in a flattening filter-free photon beams.
Methods: Flattening filter-free beam profiles (inline, crossline, and diagonals) were measured for multiple field sizes (25x25cm and 10x10cm) at 6MV on a clinical system (Truebeam, Varian Medical Systems Palo Alto CA). Profiles were acquired for baseline energy and detuned beams by changing the bending magnet current (BMC), above and below baseline. The following profile parameters were measured: flatness (off-axis ratio at 80% of field size), symmetry, uniformity, slope, and the off-axis ratio (OAR) at several off-axis distances. Tolerance values were determined from repeated measurements. Each parameter was evaluated for sensitivity to the induced beam changes, and the minimum detectable BMC change was calculated for each parameter by calculating the change in BMC that would result in a change in the parameter above the measurement tolerance.
Results: Tolerance values for the parameters were- ΔFlatness≤0.1%; ΔSymmetry≤0.4%; ΔUniformity≤0.01%; ΔSlope≤0.001%/mm. The measurements made with a field size of 25cm and a depth of d=1.5cm showed the greatest sensitivity to bending magnet current variations. Uniformity had the highest sensitivity, able to detect a change in BMC of ΔBMC=0.02A. The OARs and slope were sensitive to the magnitude and direction of BMC change. The sensitivity in the flatness parameter was ΔBMC=0.04A; slope was sensitive to ΔBMC=0.05A. The sensitivity decreased for OARs measured closer to central axis- ΔBMC(8cm)=0.23A; ΔBMC(5cm)=0.47A; ΔBMC(2cm)=1.35A. Symmetry was not sensitive to changes in BMC.
Conclusion: These tests allow for better QA of FFF beams by setting tolerance levels to beam parameter baseline values which reflect variations in machine calibration. Uniformity is most sensitive to BMC changes, while OARs provide information about magnitude and direction of miscalibration.
Funding Support, Disclosures, and Conflict of Interest: Research funding provided by Varian Medical Systems. Dr. Sasa Mutic receives compensation for providing patient safety training services from Varian Medical Systems, the sponsor of this study.
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