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A Study On Gantry Position Accuracy Using Delivered Dose Distributions for Gated Volumetric Modulated Arc Therapy

P Ravindran1*, T Balasingh1 , J Raj1 , W Woon2 , (1) Christian Medical College Hospital, Vellore, (2)The Brunei Cancer Center, Brunei, Brunei Darusalam


(Wednesday, 8/1/2018) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 4

Purpose: During respiratory-gated volumetric-modulated arc therapy (VMAT) delivery, the radiation beam turns off when the tumor is out of the gating window, however, the gantry continues to rotate before stopping due to its inertia. Hence the gantry moves back to the position where the beam turned ‘off’ and starts to rotate again with beam-on when the target enters the gating window. The accuracy of the gated VMAT dose delivery is dependent on the position of the gantry during beam-off and subsequent beam-on. Hence, additional quality assurance is needed to ensure synchronisation between the beam-on, beam-off and the gantry position.

Methods: In this work, the gated VMAT dose delivery was verified using film dosimetry (Gafchromic EBT3) and Octavius 4D by delivering the same plan with and without RPM gating. The gantry position accuracy during beam-on and beam-off was also verified using the Trajectory Log files by converting them to DICOM RT files using an ‘in-house’ Python code and comparing the dose distribution with the planned dose distribution. The phantoms used for measurements did not simulate breathing motion, as only the effect of gantry position accuracy on the delivered dose is studied. The dose distributions were compared using the Verisoft software and gamma index analyses were performed with DD/DTA criteria of 3%/3mm.

Results: In film dosimetry, the comparison between the gated and ungated dose distribution resulted in a gamma pass percentage of 91.2% with the ROI selected around the PTV. The gamma index pass percentage between the gated and ungated deliveries was found to be 99.7% when measured using 4D Octavius and 94.5% when the Trajectory Log files were compared with the planned dose distribution.

Conclusion: Clinical commissioning and periodic quality assurance program for gated VMAT delivery should include gantry position synchrony with radiation beam delivery in addition to regular QA procedures.


Not Applicable / None Entered.


TH- External beam- photons: Quality Assurance - VMAT

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