ABSTRACT
Objective:To compare the dosimetric characteristics of beams between Zap-X and G4 CyberKnife and provide reference for clinical application of Zap-X.Methods:PTW three-dimensional water tank and dosimetry diode ionization chamber were used to measure the two orthogonal off-axis ratio and field size at isocenter of 7 different collimators (5 mm, 7.5 mm, 10 mm, 12.5 mm, 15 mm, 20 mm and 25 mm) of Zap-X and CyberKnife at the water depth of maximum dose, 50 mm, 100 mm, and 200 mm. The penumbra, flatness, symmetry and field size under each parameter condition were analyzed by using PTW supporting software PTW MEPHYSTO (version 5.1). Data analysis and graph were performed using Origin 2021 software.Results:With the same collimator, the dose plateau area of Zap-X was wider than that of G4 CyberKnife, and the dose fall-off at the field edge of Zap-X system was faster. With the increase of the collimator, the penumbra of Zap-X and CyberKnife tended to become larger, and the flatness tended to become smaller, the penumbra and flatness of Zap-X were significantly smaller than those of CyberKnife. Both of them had excellent symmetry (<1%), and the symmetry results of CyberKnife (<0.39%) were better than that of Zap-X (0.99%). The accuracy of Zap-X collimator size at isocenter was better than that of CyberKnife.Conclusion:Compared with G4 CyberKnife, Zap-X system has smaller penumbra, better flatness and higher accuracy of collimator size, which is suitable for stereotactic radiosurgery.
ABSTRACT
Objective To establish a novel method of describing the off-axis ratio (OAR) characteristics of the flattening filter-free (FFF) beam.Methods The OAR curves at a depth of 1.5,5,10 and 20 cm were measured for Varian Edge,Elekta VersaHD and Tomotherapy using the water tank.The second derivatives of the OAR in the positive and negative directions were calculated.The center of the line connecting the maximum and minimum second derivatives was defined as the field edge.The distance between the left and right field edges was defined as the dosimetric field size.The OAR curve within the 80％ of dosimetric field size was fitted using the gaussian function and the fitting parameters were adopted to describe the shape of OAR.Results The calculated field size error was less than 0.11 cm and the central axis position error was less than 0.05 cm.The fitting correlation coefficient was greater than O.998.The fitting maximum error of OAR curve did not significantly alter with the depth,whereas slightly increased over the increased field size.The maximum error for a field size of 10,20,30 and 40 cm was 0.49％,0.67％,1.25％ and 2.52％,respectively.Conclusions A method which can independently and accurately describe the OAR characteristics of FFF photon beam is established for the first time,which can calculate the field size of FFF beam and fit the OAR curve of FFF beam using the gaussian function.
ABSTRACT
Objective To evaluate four detectors for the off-axis ratio profile measurements of a CyberKnife system, and provide reference and suggestions for selecting and using the correct detectors. Methods Profiles were acquired by using four detectors, PTW-60017, PTW-60018, PTW-60019 and IBA-SFD, at different depths for different collimator sizes, with the detector stem being oriented both perpendicular and parallel to the central beam axis. The differences of profiles and the influence of detector orientation on measurement result were analyzed. Results All full width at half maximum ( FWHM) of field measured by four detectors in parallel orientation was larger than that in actual field size. The deviation was increased with the size of collimator and measurement depth, with the maximum deviation of 1. 9 mm. The maximum deviation of FWHM among four detectors was 0. 2 mm. The penumbra was the smallest for IBA-SFD, and the largest for PTW-60019. The maximum deviation of penumbra was 0. 3 mm. The IBA-SFD tended to over-respond in the out-of-field region when the collimator size was larger than 30 mm. Both FWHM and penumbra in perpendicular orientation were smaller than those in parallel orientation for PTW-60017, PTW-60018 and PTW-60019, especially at 5 mm collimator. However, the trend was opposite for IBA-SFD. With the increase of collimator aperture, the difference between the right and left penumbra acquired by four detectors was increased, with more obvious stem effects. Conclusions Similar profiles were acquired by four detectors, but the detector characteristics and effects of detector orientations should be considered.
ABSTRACT
Objective To evaluate four detectors for the off-axis ratio profile measurements of a CyberKnife system, and provide reference and suggestions for selecting and using the correct detectors. Methods Profiles were acquired by using four detectors, PTW-60017, PTW-60018, PTW-60019 and IBA-SFD, at different depths for different collimator sizes, with the detector stem being oriented both perpendicular and parallel to the central beam axis. The differences of profiles and the influence of detector orientation on measurement result were analyzed. Results All full width at half maximum ( FWHM) of field measured by four detectors in parallel orientation was larger than that in actual field size. The deviation was increased with the size of collimator and measurement depth, with the maximum deviation of 1. 9 mm. The maximum deviation of FWHM among four detectors was 0. 2 mm. The penumbra was the smallest for IBA-SFD, and the largest for PTW-60019. The maximum deviation of penumbra was 0. 3 mm. The IBA-SFD tended to over-respond in the out-of-field region when the collimator size was larger than 30 mm. Both FWHM and penumbra in perpendicular orientation were smaller than those in parallel orientation for PTW-60017, PTW-60018 and PTW-60019, especially at 5 mm collimator. However, the trend was opposite for IBA-SFD. With the increase of collimator aperture, the difference between the right and left penumbra acquired by four detectors was increased, with more obvious stem effects. Conclusions Similar profiles were acquired by four detectors, but the detector characteristics and effects of detector orientations should be considered.
ABSTRACT
Objective To find the best model parameters through Monte Carlo simulation of 6 MV flattening-filter-free (FFF) beams in TrueBeam accelerator, and establish the foundation for the further study of the clinical dosimetry on 6 MV FFF X-rays.Methods Using the BEAMnrc and DOSXYZnrc codes, the percentage depth dose (PDD) and the off-axis ratio (OAR) curves of field ranges from 4 cm ×4 cm to 40 cm × 40 cm were simulated for 6 MV FFF X-ray by adjusting the incident beam energy, radial intensity distribution and angular spread, respectively.The simulation results and measured data were compared, where the optimal Monte Carlo model input parameters were acquired.Results The simulation was most comparable to the measurement when the incident electron energy, full width at half maximum (FWHM) and the spread angle were set as 6.1 MeV, 0.75 mm and 0.9°, respectively.The deviation of 1 mm (position)/1％ (local dose) could be met by the PDD of all tested field sizes and by the OAR when the fields sizes were no larger than 30 cm ×30 cm.The OAR of 40 cm ×40 cm field sizes fulfilled criteria of 1 mm (position)/1.5％ (local dose).Conclusions Monte Carlo simulation agrees well with the measurement and the proposed model parameters, which can be used for further clinical dosimetry studies of 6 MV FFF X-rays.
ABSTRACT
Objective To analyze the influence of the mean energy and the full-width of half msximum(FWHM)of incident electron beam intensity distilbution(assumed Gaussian distribution)on depth dose curves and off-axis ratios and to derive a most optimal combination of mean energy and FWHM of incident electron beam intensity distribution.Methods The study simulated 6 MV photon beam produced by Varian 600C medical linear accelerator with OMEGA/EGSnrc by matching the relative error of calculated and measured depth dose curves past depth of maximum dose and off-axis ratios at a depth of 10.0 cm in water within 2%.Results The depth dose curves were relatively insensitive to the mean energy past depth of maximum dose and the FWHM of the incident electron beam intensity distribution.Dose profiles were sensitive tO the mean energy and FWHM.The dose profiles horns decreased as the mean energy and tlle FWHM of the ineident electron beam intensity distilbution increased.The calculated value of the depth dose curves matched well with the measured value.The calculated value of the off-axis ratio was consistent with the measured value within the radiation field.However, the maximum errors of individual measurement points in the penumbra region and OUt of the field reached 18.5%.Conclusions In the field.the most optimal combination of mean energy and FWHM of incident electron beam intensitv distribution Can be derived, however,can not be derived out of the field and in the penumbra region.