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3D Gamma Test for Patient Specific Quality Assurance of Proton Pencil Beam Scanning Dose

F Pirlepesov

F Pirlepesov*, W Yao , O Ates , R Lukose , V Moskvin , L Zhao , J Farr , T Merchant , St. Jude Children's Research Hospital, Memphis, TN


SU-I-GPD-T-143 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Current practice with 2 dimensional (D) gamma test in patient specific quality assurance (QA) may not be sufficient due to strong possible dose gradient with depth in intensity-modulated proton therapy (IMPT). Our research objective is to determine Gamma test pass rate for treatment planning system (TPS) calculated 3D dose distribution against the 2D measurement without dose interpolation.

Methods: QA verification plan dose was calculated with Eclipse TPS (Varian Medical Systems, Inc., Palo Alto, CA; Eclipse v13.7.15) in a virtual water phantom (CT number = -17 HU, resolution2.05x2.05x2 mm^3). 2D dose in solid water was measured with MatriXX detector array (IBA Dosimetry, Schwarzenbruck, Germany). Monte Carlo 3D doses were simulated with QA verification plan parameters using TOol for PArticle Simulation (TOPAS) to be compared to calculated 3D dose from TPS. 3D Gamma indexes were calculated utilizing published methods (Med. Phys., 25, 656–661, 1998).

Results: 2D vs. 2D dose comparison of Gamma pass rate [min, max, average] values at 9 cm depth using OmniPro I’mRT software for 3%/3mm dose pass criterion were [94.6%, 99.2%, 98.8%] for single field optimized and [90.1%, 99.8%, 95.2%] for multi-field optimized proton beams. Results for 2D vs. 3D dose comparison using in-house code were [91.7, 100%, 97.5%] for single field optimized and [94.4%, 100%, 95.2%] for multi-field optimized proton beams. Commercial software requires an interpolation of measured dose to plan dose resolution to get the “best” result.

Conclusion: Gamma pass rates we achieved for 2D vs 3D dose comparison are similar to the commercial software’s 2D vs. 2D interpolated dose comparison. No interpolation is required by our code for measured dose which can have arbitrary pixel/voxel sizes and may be used for 2D vs. 2D, 2D vs 3D and 3D vs. 3D dose comparisons. Comparing 2D-measured dose to 3D-calculated dose increases Gamma pass rate.

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