Presentation Development of range verification system for the scanning irradiation system at NIRS

早乙女, 直也  ,  原, 洋介  ,  丹正, 亮平  ,  皿谷, 有一  ,  水島, 康太  ,  古川, 卓司  ,  白井, 敏之  ,  野田, 耕司

Purpose: Three-dimensional irradiation with a scanned carbon-ion beam has been performed from 2011 at our facility. We have been developed the rotating-gantry equipped with the scanning irradiation system. The number of combinations of beam properties to measure for the commissioning is more than 7200, i.e. 201 energy steps, 3 intensities, and 12 gantry angles. To compress the commissioning time, quick and simple range verification system is required. In this work, we develop quick range verification system using scintillator and CCD (charge-coupled device) camera and estimate the accuracy of the range verification.Materials/Methods: A cylindrical plastic scintillator block and a CCD camera were installed on the black box. The diameter of cylindrical scintillator block was 200 mm. The optical spatial-resolution of the system is 0.2 mm/pixel. The camera control system was connected and communicates with the measurement system that is part of the scanning system. The range was determined by image processing. Reference range for each energy beam was determined the 80 percent of distal dose of the depth dose distribution that were measured by a large parallel-plate ionization chamber. Total 110 mono-energy carbon beams ranging from 151.9 to 430 MeV/n were tested and compared with reference range. Measured two-dimensional images were processed by in-house program developed by c++. The common reference point of range is distal 80% of the dose distribution. However the system measures the range not with the dose distribution but with the light distribution. In order to select the best reference point on a light distribution, the authors compared two range detection methods: threshold method (TH); the threshold positions set by 30, 50, 60, 70, 80, and 90% of maximum value on the projected line are identified and difference of Gaussian (DOG) method; DOG method is widely used in edge detection field instead of Laplacian filter. Using DOG method, range position is determined by zero-crossing position in the difference between small-Gaussian smoothed data and large-Gaussian smoothed data. Sigmas for small and large Gaussian are 1 and 1.5 pixels respectively.Results: The 1 mm range difference was clearly divided. Standard deviation of discrepancy from the range measured by the ionization chamber was less than 0.1 mm. The distance between the CCD camera and beam incident point could be changed by geometrical error. A 1 mm setup error in the any direction was less than 0.2 mm range error.Conclusions: We have shown that the range of carbon pencil beam can be determined with Sub- millimeter accuracy using scintillator and CCD camera. The 80 percent of maximum value is minimized discrepancies between expected and measured ranges for carbon beam. It was supposed to be a result of the change of shape due to quenching effect. Actual impact of the geometrical error was negligible. Since the system determine the range with short time and sufficient accuracy, it seems be that the system has potential to play the daily range check system.
54th Annual Conference of the Particle Therapy Co-Operative Group (PTCOG54)

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