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Dose Rate Response of Digital Megavolt Imager Detector for Flattening Filter-Free Beams
Z Xu*, J Kim , J Han , A Hsia , S Ryu , Stony Brook University Hospital, Stony Brook, NY
Presentations
SU-I-GPD-T-265 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: The purpose of this study was to investigate the dose rate response characteristics of Digital Megavolt Imager (DMI), including panel saturation, linearity and imager ghosting effects for FFF beams.
Methods: Measurements were performed using a Varian TrueBeam equipped with DMI imager. DMI dose rate response characteristics were studied as a function of dose rate. Images were acquired at dose rates from 400 to 1400 MU/min for 6XFFF and to 2400 MU/min for 10XFFF. Line profiles and central portal doses derived from the images were analyzed and compared. The linearity was verified by acquiring 10x10 cm2 images at incremental MU irradiation from 5-500 MU. Ghosting effects were studied at different dose rates. Finally, for validation, test plans with optimal fluence were created and measured with different dose rates. All test plans were analyzed with a gamma criteria of 3%-3mm and 10% dose threshold.
Results: No panel saturation was observed from the profile comparison even with maximum dose rate of 2400 MU/min although the central portal doses showed a slight decrease from 1.013 to 1.008 cGy/MU when dose rate was increased from 400 to 1400 MU/min for 6XFFF, and from 1.020 to 1.009 cGy/MU when dose rate was increased from 400 to 2400 MU/min for 10XFFF. The linearity was better than 0.5% in the range of 5-500 MU for all energies. The residual image was extremely small 0.03±0.08% (1 SD) after 500 MU delivery. The test plans showed gamma index decreased from 100% to 97.8% for 6XFFF when dose rate was increased from 400 to 1400 MU/min and from 99.9% to 91.5% when dose rate was increased from 400 to 2400 MU/min for 10XFFF.
Conclusion: The Portal Dosimetry system for the TrueBeam with DMI imager can be used for IMRT and VMAT pretreatment QA verification for FFF energies.
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