R. Tansho

Experimental verification of gain drop due to general ion recombination for a carbon‐ion pencil beam

PURPOSE Accurate dose measurement in radiotherapy is critically dependent on correction for gain drop, which is the difference of the measured current from the ideal saturation current due to general ion recombination. Although a correction method based on the Boag theory has been employed, the theory assumes that ionized charge density in an ionization chamber (IC) is spatially uniform throughout the irradiation volume. For particle pencil beam scanning, however, the charge density is not uniform, because the fluence distribution of a pencil beam is not uniform. The aim of this study was to verify the effect of the nonuniformity of ionized charge density on the gain drop due to general ion recombination. METHODS The authors measured the saturation curve, namely, the applied voltage versus measured current, using a large plane-parallel IC and 24-channel parallel-plate IC with concentric electrodes. To verify the effect of the nonuniform ionized charge density on the measured saturation curve, the authors calculated the saturation curve using a method which takes into account the nonuniform ionized charge density and compared it with the measured saturation curves. RESULTS Measurement values of the different saturation curves in the different channels of the concentric electrodes differed and were consistent with the calculated values. The saturation curves measured by the large plane-parallel IC were also consistent with the calculation results, including the estimation error of beam size and of setup misalignment. Although the impact of the nonuniform ionized charge density on the gain drop was clinically negligible with the conventional beam intensity, it was expected that the impact would increase with higher ionized charge density. CONCLUSIONS For pencil beam scanning, the assumption of the conventional Boag theory is not valid. Furthermore, the nonuniform ionized charge density affects the prediction accuracy of gain drop when the ionized charge density is increased by a higher dose rate and/or lower beam size.

SU-F-J-190: Time Resolved Range Measurement System Using Scintillator and CCD Camera for the Slow Beam Extraction

PURPOSE To investigate the time structure of the range, we have verified the rang shift due to the betatron tune shift with several synchrotron parameters. METHODS A cylindrical plastic scintillator block and a CCD camera were installed on the black box. Using image processing, the range was determined the 80 percent of distal dose of the depth light distribution. The root mean square error of the range measurement using the scintillator and CCD system is about 0.2 mm. Range measurement was performed at interval of 170 msec. The chromaticity of the synchrotron was changed in the range of plus or minus 1% from reference chromaticity in this study. All of the particle inside the synchrotron ring were extracted with the output beam intensity 1.8×108 and 5.0×107 particle per sec. RESULTS The time strictures of the range were changed by changing of the chromaticity. The reproducibility of the measurement was sufficient to observe the time structures of the range. The range shift was depending on the number of the residual particle inside the synchrotron ring. CONCLUSION In slow beam extraction for scanned carbon-ion therapy, the range shift is undesirable because it causes the dose uncertainty in the target. We introduced the time-resolved range measurement using scintillator and CCD system. The scintillator and CCD system have enabled to verify the range shift with sufficient spatial resolution and reproducibility.

TH-CD-BRA-07: Range Verification System Using Scintillator and CCD Camera for the Scanning Irradiation System

Purpose: To compress the quality assurance time for the scanned carbon-ion beam system, we have been developed the novel range verification system using scintillator and CCD (charge-coupled device) camera. Methods: A cylindrical plastic scintillator block and a CCD camera were installed on the black box. 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. Carbon beams ranging from 151.9 to 430 MeV/n were tested and compared with reference range. The common reference point of range is distal 80% of the dose 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 the range from 10 to 90% of maximum value on the projected line are identified and difference of Gaussian (DOG) method. Using DOG method, range position is determined by zero-crossing position in the difference between small-Gaussian smoothed data and large-Gaussian smoothed data. Results: The 1 mm range difference was clearly detected. Standard deviation of discrepancy from the range measured by the ionization chamber was less than 0.1 mm. A 1 mm setup error in the any direction was less than 0.2 mm range error. Conclusion: We have shown that the range of carbon 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. 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.