(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: The purpose of this study was to compare the results of several different pre-treatment patient specific QA devices used on the Varian Halcyon Linear Accelerator (Halcyon) – including Varian portal dosimetry, point dose measurement in a homogenous slab phantom, and PTW Octavius4D with PTW 1500 detector.
Methods: A total of 185 treatment plans for prostate, prostate and nodes, prostate bed, rectum, anus, abdomen, brain, and head & neck were planned using varying numbers of arcs ranging from 2-6 for VMAT plans, as well as 7-9 field IMRT plans. Verification plans were created within the Eclipse treatment planning system (TPS) on phantoms ready for physics quality assurance.For the array devices and portal dosimetry, delivered results were then compared with exported results from the TPS using a 2%/2mm and 3%/3mm gamma evaluation (3D for PTW Octavius4D and 2D for Varian portal dosimetry) with a 10% threshold, referenced to the global dose maximum. For point dose measurements (using a PTW 3D pin point chamber), delivered results were compared using local dose difference expressed as a percentage.
Results: Point dose measurements, per plan, averaged 1.0% with a standard deviation of 1.2%.Varian Portal dosimetry results ranged from 87.7% - 100.0% and 99.0% – 100.0% for 2%/2mm and 3%/3mm respectively. PTW Octavius4D results ranged from 64.7% - 98.2% and 90.8% - 99.9% for 2%/2mm and 3%/3mm respectively. This also highlighted a trend of a decreasing gamma pass rate both as the number of VMAT arcs increased, as well as when the gantry rotation speed exceeded that of the Octavius4D phantom.
Conclusion: Point dose measurements, Varian portal dosimetry, and PTW Octavius4D were all used successfully to conduct pre-treatment patient specific QA on a variety of plans delivered using a Varian Halcyon linear accelerator. These results were compared and conclusions drawn.
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