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Objective:To evaluate the volumetric modulated arc therapy (VMAT) dose verification of cervical cancer based on γ rule and dose volume histogram (DVH) and to perform correlation analysis between the evaluation results and the dose differences.Methods:Twenty cervical cancer VMAT plans were selected and performed on TrueBeam Linac. The delivered point and surface dose was measured by FC-65 G and ArcCheck and the results were compared to those calculated by Eclipse. The dose of patients was reconstructed by 3DVH. Then, differences between the reconstructed and plan value of D mean, D 95%, D 98% and D 2% of PTV, V 20Gy of left and right femoral head, V 40Gy of rectum, D 1cm 3 of cord, D 98%, D 2% and D 50% of the 50% prescription iso-dose volume (IDV), were evaluated and 3-dimensional (3D) γ was assessed for each organ. Lastly, Pearson’s correlation coefficient was used to analyze the relationship between point dose difference, 2D γ pass-rate (γ%), γ mean and 3D γ% of each organ and the dose difference. Results:Small differences were found between the point dose measured, reconstructed and the plan value. Differences between D mean of PTV, all dose parameters of IDV and plan values were all within 3% and V 40Gy of rectum showed the largest difference. As for the 3D γ%, the maximum pass rate was found for the left and right femoral head and the maximum variance for cord D 1cm 3. There was a moderate correlation between measured and reconstructed point dose deviation and dose difference of each organ, while no significant correlation was found for 2D γ%. Strong correlation was found between 3D γ% of target and D 50% of PTV/IDV and no correlation was found for other organs. Conclusion:The performance of both γ-and DVH-based evaluation can reveal dose error for dose verification, but both of them have some limitations and should be combined in clinical practice.
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Objective:To perform testing and clinical application of a volumetric-modulated arc therapy (VMAT) dosimetry verification system based on three-dimensional dose reconstruction of patient anatomical structures.Methods:ArcCheck array calibration was performed. Then, 200 MU was delivered with a 10 cm×10 cm field when the source to center of ArcCheck was 100 cm to calibrate the absolute dose and the dose was simultaneously measured by a FC65-G detector in the center of the ArcCheck. The absolute dose calibration value or the CT value of ArcCheck was adjusted to minimize the differences between the planning and measurement values of FC65-G, reconstructed value by 3DVH and reconstructed percent depth dose by 3DVH. 10 lung and 10 cervical cancer VMAT cases were selected and measured by ArcCheck and FC65-G under the delivery of a TrueBeam LINAC. The three-dimensional doses of all cases were reconstructed by 3DVH and compared with the planning and measurement values.Results:Different array calibration files of ArcCheck exerted different effect upon the two-dimensional dose measured by ArcCheck and three-dimensional dose reconstructed by 3DVH. The optimal reconstructed dose was obtained when self-calibration file was adopted and 249.96 cGy was regarded as the absolute dose calibration value. The deviations of the mean dose (D mean) and D 95% of the target were within ±4.2% and parameters of some organs at risk significantly differed compared with the reconstructed and planning dose for all cases. A negative mean point dose difference was obtained and the reconstructed dose was closer to the measured value. The γ-passing rate of the target for some cases was low, the proportion of regions irradiated by 50% prescription dose was slightly higher and the proportion of other organs was relatively high. Conclusion:The 3DVH model can be accurately established and tested with the acceptance test method in the present study, which can provide detailed information for dose verification.
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Objective:To evaluate the combined effect of an trajectory log field based(LBF)and two commercial dose reconstruction systems on volume-modulated arc therapy(VMAT)dose verification of lung cancer.Methods:An in-house program was developed to introduce errors in trajectory log of TrueBeam to the origin plan and recalculate the dose of the error plan in treatment planning system(TPS). A total of 18 lung cancer cases treated by two-arc VMAT were selected to perform on LINAC and measured by ArcCheck simultaneously. Then, the reconstructed doses were obtained by 3DVH. The mode of reconstruction was calculated by LFB and Compass. Five of the 18 cases were performed on LINAC two times in four hours and measured by ArcCheck to evaluate the stability of the TrueBeam performance. The 18 plans were recalculated and performed on LINAC with a solid water phantom with 5 cm build-up, 4 cm back scattering thickness and a FC65-G detector in the center. The measured dose by detector was compared with the reconstructed dose by three systems.Results:TheTruebeam performance was stable. For all of the 18 cases, the point dose measured by FC65-G and reconstructed by three systems had a deviation of less than 2% to the TPS calculated. For all of the organs reconstructed by LBF and most organs reconstructed by 3DVH and Compass, the γ pass rate between them and TPS all exceeded 90% under all criteria, as well as the ArcCheck measured results. For all the organ dose difference between reconstructed and TPS, LBF system had the smallest difference, followed by the Compass system except the lung, and the 3DVH had the highest difference.Conclusions:LBF, 3DVH and Compass can reflect the VMAT dose verification results of lung cancer from different perspectives. The combined application of three systems can demonstrate the verification results in an intuitive manner, which is beneficial for subsequent analysis.
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Objective:To evaluate the multi-leaf collimator (MLC) performance of TrueBeam accelerator using trajectory log files.Methods:All tests were performed 5 times under different gantry-collimator angle combination. The 1 mm picket fences were constructed by static or dynamic MLC. The control ability for small-field accuracy of accelerator was evaluated. Repeatability was assessed by MLC repeat motion. The movement performance of difference velocities along one direction and the opposite direction were evaluated via a 1 cm picket fences which slipped from -7 cm to 7 cm with a uniform velocity and stopped or immediately back at 7 cm. The MLC performance in a complex program was evaluated by a cross movement test.Results:Both the static and the dynamic picket fences yielded high accuracy. The deviation spectrums of MLC in different gantry angle were consistent, however, an absolute difference of 0.001 1 mm was found. For uniform velocity movement tests with 0°gantry, the RMSE of MLC was increased from 0.015 0 mm to 0.059 8 mm when the speed was accelerated from 5 mm/s to 25 mm/s. Similar results were obtained in non-zero gantry angle. The "overspeed" effect caused by the direction change movement of MLC was less obvious than that caused by speed changed from zero to a uniform velocity movement state. There was no significant change in speed before and after the MLC crossing. The MLC speed fluctuated around the set value, which was independent of the gantry angle.Conclusion:A method for evaluating the performance of MLC using trajectory log files is established, which can evaluate the MLC performance of TrueBeam accelerator and be used for MLC rapid quality control in clinical practice.