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A Novel Technique for VMAT QA with EPID in Cine Mode On Varian TrueBeam

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B Liu

B Liu1,2*, J Adamson2, A Rodrigues2, F Zhou1, F Yin2, Q Wu2, (1) Beihang University, Beijing, China, (2) Duke University Medical Center, Durham, NC

WE-G-108-8 Wednesday 4:30PM - 6:00PM Room: 108

Purpose: Volumetric modulated arc therapy (VMAT) is a relatively new treatment modality for dynamic photon radiation therapy. Pre-treatment quality assurance (QA) is necessary and many efforts have been taken to apply electronic portal imaging device (EPID) based IMRT QA methods to VMAT. It is important to verify the gantry angles during delivery as this is new variable that is also modulated in VMAT. The purpose of this study is to develop and validate a new technique that is accurate and efficient to perform VMAT QA using EPID.

Methods: The method utilized EPID cine mode and was tested on Varian TrueBeam in research mode. The cine images were acquired during delivery and converted to dose matrices after profile correction and dose calibration. Sub-arc corresponding to each cine image was extracted from the original plan and its portal image prediction was calculated. Several analyses were performed including 3D γ analysis (2D images plus gantry angle axis), 2D γ analysis, and other statistical analyses. The method was applied to 21 VMAT beams of 6MV photons and the accuracy of cine image information was investigated. Furthermore, this method's sensitivity to machine delivery errors was studied.

Results: The pass rate (97.8±1.1%) for 3D γ analysis was comparable to that from Delta⁴ (99.9±0.1%) under similar criteria (3%, 3mm, 5% threshold and 3°angle to agreement). The recorded gantry angle and start/stop MUs were found to have sufficient accuracy for clinical QA. Machine delivery errors can be detected through combined analyses of 3D γ index, gantry angle, and percentage dose difference.

Conclusion: We have developed and validated a QA technique that can simultaneously verify the gantry angle and delivered MLC fluence for VMAT treatment. The method is efficient and its accuracy is comparable to other methods.

Funding Support, Disclosures, and Conflict of Interest: B Liu's work was supported by the Chinese Scholarship Council.

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