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Using the Varian Portal Dosimetry Scripting API to Establish Action Levels for IMRT QA

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J Paisley

J Paisley1*, J Seger-Paisley2 , (1) Coastal Carolina Radiation Oncology, Wilmington, NC, (2) New Hanover Regional Medical Center, Wilmington, NC


SU-I-GPD-T-254 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Use the Portal Dosimetry Scripting Application Programming Interface (PDSAPI) to establish action levels for portal dosimetry QA.

Methods: An application was written with the PDSAPI to evaluate portal dosimetry QA of 477 portal dose images (PDIs) from 157 patients following the commissioning of Varian Medical Systems Portal Dosimetry v13.6. PDIs were acquired on two Varian Clinac iX linear accelerators equipped with as500 portal imagers. During the initial period from April – September 2016, IMRT QA was performed using the gamma analysis method with 3 % dose difference (DD), 3 mm distance to agreement (DTA) and 10% dose threshold (TH). Initial passing criteria was that % area gamma < 1 be > 95%. The PDSAPI application extracted the max gamma (γmax), average gamma (γavg), and area gamma <1 (γarea<1). Then, it created a transient analysis for each PDI using 3% DD, 2 mm DTA and 10% TH. Finally, the application computed the means and standard deviations (SDs) of γmax, γavg, and γarea<1 for both the existing and transient analyses. Action levels were reset to 2*SD of the mean values obtained during the initial period. These updated action levels were applied to IMRT QAs done during the trial period from October – December 2016 with 3% DD, 2 mm DTA, and 10% TH. The PDSAPI application extracted the means and SDs of γmax, γavg, and γarea<1 for patients with the new action levels. These results were used to validate the chosen action levels.

Results: 97% of the parameters evaluated were within 2 SDs of the mean. Some large values of maximum gamma indicated plans that had leaf junctions parked inside the treatment field.

Conclusion: The PDSAPI is capable of advanced data mining to rapidly identify trends and anomalies for portal dosimetry QA.

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