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Error Detection in End-To-End Test for CT-Based HDR Brachytherapy Using a Normoxic N-Vinylpyrrolidone Polymer Gel Dosimeter

H Tachibana1*, Y Watanabe2 , S Mizukami2 , T Maeyama2 , T Terazaki3 , R Uehara1 , T Akimoto1 , (1) National Cancer Center Hospital East, Kashiwa, ,(2) Kitasato University, Sagamihara, ,(3) Yokohama City Minato Red Cross Hospital, Yokohama,


(Monday, 7/15/2019) 1:15 PM - 1:45 PM

Room: Exhibit Hall | Forum 6

Purpose: We developed end-to-end (E2E) test of dose and source position using a gel dosimeter for the CT-based high-dose-rate brachytherapy and evaluated the feasibility of error detection.

Methods: A normoxic N-vinylpyrrolidone-based polymer gel (VIPET gel) was filled in a cylindrical acrylic container. The dose response was evaluated on the basis of the transverse relaxation rate (R2) measured by magnetic resonance scanning. A treatment plan was generated using Oncentra planning system (Elekta, Sweden). Planar gamma images on a coronal plane were calculated using the measured dose and calculated doses from the TPS under three various scenarios: the plan of the irradiation (defined as no error), plans with either simulated random or systematic source position errors and plans with air kerma strength setting errors. The systematic displacements were ±2 mm respectively. The magnitude and direction of the random displacement was added to each source position with a 2.7-mm standard deviation. The range of the air kerma strength error was from 2% to 20%.

Results: The plan with no error shows ≥ 95% gamma pass ratio with 2-3 mm/1-5% gamma criteria. The positional errors were found with ≤ 95% pass ratio when the criteria were 1-2 mm/1-5%. Pass ratios for random error were decreased by 10% compared to those for systematic error. The strength errors with 10% and 20% were found with ≤ 95% pass ratio when the criteria were 2-3 mm/1-3%. The pixels with ≥ 1 gamma value were appeared on the axis of the source transport for the positional errors and in the peripheral region for the dose errors respectively.

Conclusion: The feasibility of error detection in E2E test using the gel dosimeter has been demonstrated. This methodology represents a substantial step forward for CT-based brachytherapy QA and the potential to distinguish between different types of errors.


Gel Dosimeter, Image-guided Therapy, Ir-192 Source


TH- Brachytherapy: Calibration & Quality Assurance

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