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Real-Time Verification of VMAT Delivery by An Automated Beam Monitoring System

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M Farrokhkish

M Farrokhkish*, M Islam , R Heaton , B Norrlinger , D Jaffray , Princess Margaret Cancer Centre, University Health Network, Toronto, ON


SU-I-GPD-T-225 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess the performance of a commercial beam monitoring system for real-time validation of VMAT treatment.

Methods: The Integral Quality Monitor (IQM) system consists of a spatially sensitive area ion chamber mounted to the Linear Accelerators’ (Linac) collimator, and a calculation algorithm to predict the signal based on beam parameters. An on-board electrometer and accelerometer enable ionization signal measurement for each beam segment. Initial performance of IQM was assessed by measuring the reproducibility of segment-by-segment and cumulative signals of VMAT fields on Varian TrueBeam Linacs. The measured reproducibility was evaluated based on the information derived from the Trajectory Log File (TLF). Recognizing the significant variance across the segment boundary in beam delivery by the Linac and IQM measurement, a Signal Difference tolerance and Angle-to-Agreement (ATA) limit was explored for validation. Subsequently, similar criteria were used for evaluating the calculation algorithm against measured signals. A total of 32 Head & Neck VMAT treatment plans created in Pinnacle (Philips Medical) were used for this study. Sensitivity of the system in detecting significant errors was also evaluated.

Results: VMAT reproducibility was assessed by creating a reference from the averaged signal for five consecutive runs of each field. A 5% signal difference and ATA of 0.5 degree in gantry angle in the segment boundary produced ~90% pass rate. Similar criteria for comparison between the measured and calculated signals show a pass rate between 80 to 95%. The agreement for the cumulative total signals was found to be within ±3%. Each VMAT field produced a unique signature, distinguishable from delivery with clinically significant errors.

Conclusion: IQM system provides a platform to detect beam delivery errors with high sensitivity and specificity. Its stability and automated operations makes it suitable for efficient quality assurance (QA) of VMAT treatment beams either for pre-treatment or during treatment delivery.

Funding Support, Disclosures, and Conflict of Interest: Some of the authors have a royalty relationship with the manufacturer of the IQM system.

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