Program Information
Influence of the Applied Voltage On the Ion Recombination Correction Factor Using the Two-Voltage Technique in Flattening Filter-Free Beams
r yada1*, (1) Kobe University Graduate School of Medicine, Kobe City, Hyogo, n hayashi2*, (2) Fujita Health University, Toyoake City, Aichi, h akasaka1*, (1) Kobe University Graduate School of Medicine, Kobe City, Hyogo,n mukumoto1*, (1) Kobe University Graduate School of Medicine, Kobe City, Hyogo,r sasaki1*, (1) Kobe University Graduate School of Medicine, Kobe City, Hyogo
Presentations
WE-RAM2-GePD-TT-3 (Wednesday, August 2, 2017) 10:00 AM - 10:30 AM Room: Therapy ePoster Theater
Purpose: The two-voltage technique is commonly used for the computation of the ion recombination correction factor. In addition, the use of the two-voltage technique is recommended by AAPM TG-51 addendum. However, the range of the electrometer applied voltage can affect the ion recombination correction factor using the two-voltage technique. It is expected that flattening filter-free (FFF) beams are likely to be affected since FFF beams have high dose per pulse. The purpose of this study is to investigate the influence of the range of the electrometer applied voltage on the ion recombination correction factor using the two-voltage technique in FFF beams.
Methods: We used both 6 MV and 10 MV conventional flattening filter (FF) beams and FFF beams on a TrueBeam linear accelerator (Varian). The measurement was conducted using a CC13 chamber (IBA) and Blue-Phantom2 water tank (IBA). The measurement setup was source-to-axis distance (SAD) of 100 cm and a 10 × 10 cm2 field. We varied the depth (5, 10, 15, 20 cm) and the electrometer applied voltage (400/200 V, 400/100 V, 300/100 V, 200/100 V, 200/50 V, 100/50 V).For chamber readings, five minutes were taken to reach equilibrium after changing applied voltages. The ion recombination correction factor was computed using the equation from TRS-398 (IAEA).
Results: The correction factor was dependent on the setting of the high applied voltage and there is little influence of the low applied voltage. As the high applied voltage is high, the correction factor was low. In comparison between 400/100 V and 300/100 V, the differences of the correction factor in 6 MV FF, 6 MV FFF, 10 MV FF, 10 MV FFF were up to 0.15%, 0.33%, 0.15%, 0.56%, respectively.
Conclusion: In FFF beams, the range of the electrometer applied voltage can affect the ion recombination correction factor using the two-voltage technique.
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