(Saturday, 4/7/2018)
Room: Foyer
Purpose: To develop a procedure to efficiently measure electron Râ‚…â‚€ utilizing the Sun Nuclear IC-Profiler Quad Wedge accessory for non-standard energies. Currently, methods to calculate Râ‚…â‚€ are only provided for energies 6, 9, 12, 15, 16, 20 and 22MeV.
Methods: The energies investigated have been tuned off the standard settings to achieve specified R₉₀ values. PDDs were measured using a water tank for nominal electron energies of 7, 9, 10, 11, 13, 16, and 20MeV with 1, 2, 3, 4 and 5mm of PVC placed in a 25x25cm² cone as close to the linear accelerator head as possible. R₅₀ values were calculated for overlapping energies (9, 16, 20MeV) and compared to water tank scans. Plots of the water tank measured R₅₀ versus the AreaRatio (a metric used by Sun Nuclear) for each energy were fitted with a quadratic. R² values were used to determine the accuracy of the fitted regression. Lastly, IC-Profiler calculated R₅₀ values were compared to solid water spot measurements.
Results: The average distance-to-agreement for 9, 16, and 20MeV using the coefficients provided by Sun Nuclear was -0.17±0.83mm. Quadratic fits using the PVC range shifters yielded the following R² values: 7MeV=0.8408, 9MeV=0.9143, 10MeV=0.9987, 11MeV=0.9954, 13MeV=0.9988, 16MeV=0.9515 and 20MeV=0.8898. The average DTA for these energies calculated by the IC-Profiler was 0.02±0.43mm versus 0.96±0.48mm for solid water spot measurements.
Conclusion: All of the measurements suggest the sensitivity of the IC-Profiler Quad Wedge is capable of meeting the 2%/2mm tolerance suggested by TG-142. Although still much improved from solid water spot check measurements, the uncertainty using PVC sheets (0.43mm on 1 machine) to shift Râ‚…â‚€ was higher than what was reported by Sun Nuclear (0.18mm on 11 machines) who changed the bending magnet current. A comprehensive analysis, including multiple matched machines is ongoing.
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