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Objective To determine the optimal electron beam energy at different field size through a Monte Carlo-based simulation of the therapy head of Varian X 6 MV linear accelerator so as to study the influence of radial intensity on depth dose.Methods Firstly,keeping the radial intensity unchanged for the field of interest while changing electron beam energy,compassion was carried out of calculated percentage depth doses between measured values.Thus,the optimal energy was identified for this field size.Then,the obtained energy was set the optimal value to study the radial intensity influence on the depth doses.Results The optimal electron energy for 4 cm ×4 cm,10 cm × 10 cm,20 cm × 20 cm and 30 cm × 30 cm field sizes was 5.9,6.0,6.3 and 6.4 MeV respectively.Changes in radial intensities resulted in negligible changes in percentage depth doses for4 cm ×4-cm and 10 cm × 10 cm fields,but led to observable discrepancy for 20 cm × 20 cm and 30 cm × 30 cm fields.Conclusions The optimal electron energies for different field sizes are slightly different.Change in radial intensity distribution has significant influence on the depth dose for large field.To improve simulation accuracy,the field size needs to be taken into consideration in determining the electron beam energy and radial intensity distribution.
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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.
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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.
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Objective To develop a measurement method of dosimetric parameters for Hi-ART tomotherapy unit. Methods Percentage depth doses and beam profiles were measured using the dedicated mini water phantom, and compared to the results of 6 MV X-ray from Primus accelerator. Following the AAPM TG51 protocol, absolute dose calibration was carried out under SSD of 85 cm at depth of 1.5 cm for field of 5 cm ×40 cm. The output linearity and reproducibility were evaluated. The output variation with the gantry rotation was also investigated using 0.6 cm3 ion chamber in cylindrical perplex phantom and on-board MVCT detectors. Leaf fluence output factors were quantified for the leaf of interest and its adjacent leaves.Results The buildup depth was around 1.0 cm. The PDD values at 10 cm for Hi-ART and Primus were 59.7% and 64.7%, respectively. Varying with the field width, the lateral and longitudinal beam profiles were not so homogeneous as the Primus fields. The measured dose rate was 848.38 cGy/min. The fitted lint(sec) ,with a relative coefficient of 0. 999. The maximum deviation and standard deviation of output were 1.6% and less than 0.5% ; The maximum deviation and standard deviation of output changed by gantry angle were 1.1% and 0.5 % , respectively. Leaf fluence output factors did not increase significantly when leaves were opened beyond the two adjacent leaves. Conclusions Hi-ART Tomotherapy unit has a very high dose output and inhomogeneous beam profiles owing to its special design of the treatment head. This may be useful in dose calculation and treatment delivery.
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Objective To fit the functional relation between Percentage Depth Dose and depth by Newton Interpolation. Methods After analyzing the data of Percentage Depth Dose from different manufacturers' linear accelerators, the average of Percentage Depth Dose with same depth was calculated, and then the average data was determined. The interval was set and the interpolation node was selected for simulating the cubic polynomial with PDD and depth. Results Comparing the calculated values by the function with the measured ones, the error was less than 1%. Conclusion The function simulated with Newton Interpolation is applicable in routine clinical radiotherapy and research.