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Commissioning and Validation of the TrueBeam AS1200 Portal Imager for Patient Specific, Pre-Treatment QA of VMAT Plans

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D Wieczorek

D Wieczorek*, V Mishra , A Gutierrez , Miami Cancer Institute, Miami, FL


SU-I-GPD-T-262 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The purpose of this study was to commission the 43x43 cm² Varian aS1200 MV detector-based dosimetry system for TrueBeam accelerators with both millennium-120 and HD120 MLCs. Furthermore, validation of the aS1200 imager for use in pre-treatment verification of VMAT plans using 6X, 15X, 6FFF and 10FFF beams was performed.

Methods: The aS1200 MV detector-based dosimetry system was commissioned on two beam-matched TrueBeam with millennium 120 MLC and one TrueBeam STX with HD120MLC. Methodology for the commissioning and acceptance was developed using the Varian recommended protocols and in-house modifications. Diagonal profiles used for the 6X and 15X beams were measured at 5cm and 10cm depth, respectively, instead of the usual dmax depth for better agreement of the flat-fluence fields. Ten VMAT treatment plans of different energy, site, and dose per fraction including SBRT were generated in Eclipse for all three linacs. Predicted portal dose for each plan was then calculated on the three TrueBeams and measured. Mapcheck2 and ArcCheck composite plan measurements were also conducted for comparison.

Results: Results show excellent agreement among the two TrueBeams and TrueBeam STX. The agreement between the predicted portal dose and portal dose measurements as quantified by the gamma index were 99.6% (0.4% SD) for the three TrueBeams using 3%, 3mm, 10% threshold global gamma and 98.8% (1.1%SD) using 2%, 2mm, 10% threshold gamma. MapCheck2 and ArcCheck measurements show good correlation with the portal dosimetry results with >95% results for 3%, 3mm, 10% threshold gamma.

Conclusion: A comprehensive commissioning and validation study was performed for the aS1200 portal imager for pre-treatment verification of VMAT plans. The aS1200 portal imager was found to be a robust tool to verify the evaluated VMAT plans including small stereotactic fields with FFF beams.

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