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Dosimetric Verification of Gated IMRT Utilizing An In-House Developed Dynamic QA Device

C Liu

C Liu*, K Xie , D Pearson , N Sperling , E Parsai , University of Toledo Medical Center, Toledo, OH


SU-F-FS2-3 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: Four Seasons 2

Purpose: Compared with internal target volume intensity modulated radiation therapy (ITV-IMRT), gated IMRT can spare more of the surrounding normal tissue during the radiation treatment. For gated treatment, it has been a concern that the “actual” dose distribution is hard to be measured because the target and the surrounding tissues are moving along the respiratory motion while the measurement device is static. The purpose of this study is to develop a dynamic QA device and verify the dosimetric outcome of gated IMRT plans.

Methods: A Patient specific respiratory motion waveform as recorded by Varian RPM system during the patient’s 4D CT scan was used to define 3 phase-based gating windows centered about peak inspiration: 30%, 50%, and 70%.. IMRT plans were generated for each, as well as free-breathing, using identical beam configurations. A commercially available respiratory motion phantom has been modified to simulate patient’s intra-fraction organ motion on 3D diode-phantom. All plan sets were delivered with appropriate gating to the dynamic QA device, and the measured data was analyzed using the SNC software.

Results: The average gamma passing rate (3%, 3mm) was 89.8% for 30% gating window, 84.3% for 50% gating window,79.5% for 70% gating window, and 74.8 % for non-gated plan. The measured data was calculated on patient anatomy using the 3DVH software. Reviewing of measured data indicates that most of the dose discrepancies were located at the path of organ motion, and the tighter the gating window, the more accuracy in plan delivery.

Conclusion: By using the dynamic QA device, the results indicate that with tighter gating window a higher level of accuracy in beam delivery is achieved. Smaller gating window results in creating a smaller dose penumbra around the PTV and significantly less dose to surrounding structures.

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