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Objective To investigate the dosimetric effect of accelerator gantry rotation angle errors, collimator and multileaf collimator ( MLC) leaf position errors on volumetric-modulated arc therapy ( VMAT) for cervical cancer. Methods A total of 10 patients with cervical cancer were selected. The plan. Trail file of each clinical plan was extracted from the Pinnacle3 V9. 2 planning system of USA Philips, then the operating parameters of tach control point were read and modified by Matlab programs, and thus the operating error of the accelerator was simulated. Results In this paper, it was discovered that systematic accelerator gantry rotation angle errors, systematic collimator position errors and systematic MLC shift errors which led to the maximum changes of the PTV dose limit were 0. 16％, 0. 46％ and 0. 57％, respectively, and the maximum changes of the dose limit of organs at risk ( OAR) were 0. 38％, -1. 32％ and -0. 44％, respectively. When the systematic MLC gap width errors were ± 0. 5, ± 1 and ± 2 mm, respectively, the maximum changes of PTV dose were 2. 11％, 3. 04％ and 6. 03％, respectively, while the maximum changes of the OAR average dose were 2. 17％, 3. 92％ and 7. 97％, respectively. Furthermore, the dose limits of PTV and OAR showed a strong linear correlation with MLC open or close errors(t=21. 201～90. 562,P<0. 05). If actual errors of each parameter of accelerator were introduced, the maximum changes of PTV and OAR dose limits were 0. 16％ and 1. 30％, respectively, and conformity index (CI) and homogeneity index (HI) were barely changed. Conclusions No significant effect was found for systematic accelerator gantry rotation angle errors, systematic collimator position errors and systematic MLC shift errors for cervical cancer VMAT patients. However, there is a high sensitivity to dose distribution for MLC open or close errors. Therefore, it is necessary to pay more attention on the quality control of the accelerator running in particular MLC position errors to ensure the therapeutic accuracy.
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Objective To establish a new method to analyze the position accuracy of multileaf collimator (MLC) in the dynamic mode.Methods The MLC test sequence was created in a field,where intentional leaf positional errors ranging from 0.1 to 1 mm per centimeter were introduced.In order to establish the relationship between the ion chamber readings and leaf position,whose slope indicated the leaf position error per centimeter,a two-dimensional ion chamber array was used to measure absorbed dose while leaves were moving at dose rates of 100,300 and 600 MU/min,respectively.For routine test,leaf position error was easily found via dose profile in y direction of the field created by dynamic leaves,where the position error could be quantitatively calculated as the slope of absorbed dose line of x direction of the same field.Results The error of 0.2 mm or more per centimeter was obviously shown through y dose profile.The calibration curve was linear at different dose rates.At 600 MU/min,a 0.1 mm leaf position error corresponded to a slope variation of 0.74％,and the differences between the tested errors and the introduced errors were within 0.1 mm.Conclusions The simple and reliable method is helpful to establish the intensity modulated radiation therapy (IMRT) quality control (QC) system.
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Objective To verify the daily delivery accuracy of volumetric-modulated arc therapy (VMAT) and achieve the process quality control (PQC) of linac using statistical process control (SPC)technology.Methods The log files of all treatments were taken out from the linac system.An in-house software which created by Matlab 7.14 was used to analyze the daily parameters accuracy and the mechanism and dose delivery accuracy of the linac for each VMAT,and an daily evaluation report was automatically created for the linac.The stability of the linac and the deviation of the delivery accuracy for various cancer sites were also analyzed.To achieve the process quality control of hnac using SPC technology,the control limit of each parameter was calculated by Johnson alternation and the single value control charts were drawn.Results 76 patients mainly with nasopharyngeal carcinoma,cervical carcinoma,rectal cancer and laryngeal cancer,got the treatment daily with VMAT technology,and a total of 2 446 arcs were delivered by the linac in continuous 16 days.The gantry angle error,y and x collimator position error,MLC leaf position error and dose delivery error were 0.49°,0.09 mm,0.38 mm,0.31 mm,0.05 MU,respectively.Compare to the nasopharynx,uteri,rectal and larynx cancer,it can be seen that the variation factors of dose delivery and gantry angle errors for various cancer sites were higher,which was 8.10％ and 4.54％,respectively.Through the process quality control of the linac,it was found that all the parameters were in control.While some abnormal points arose (the error was greater than UCL),and the ratio of the gantry angle which was out of control (0.45％) was the highest in all parameters.Conclusions linac delivery accuracy could be monitored and verified during the whole VMAT treatment for each patient,and the daily running condition of linac could be monitored.Through the introduction of SPC technology and the control charts,the process quality control of linac in VMAT could be achieved.It is a financial and valid method for the daily quality assurance of linac.
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Objective To study the method of measuring the position accuracy and the step distance accuracy of afterloading system with192Ir source by using flushing-free film.Methods The position accuracy and the step distance accuracy of a China-made afterloading system with192Ir source was measured by using GAFCHROMIC (R) EBT3 flushing-free film.The film was scanned to proper image format,required by dose analysis software,by EPSON PREFACTION V700 PHOTO scanner.Then images are analyzed by using film dose analysis software in SNC Patient 5.2.Results With focus on the center of active section of source,the position accuracy of this afterloading system with192Ir source was-0.75 mm.Using film analysis could make the step point to tell apart if the step distance was 5 mm away by the method of film analysis,but couldnot make it to tell apart if the step distance was 2.5 mm away.The 2.5 mm step distance accuracy could be judged if the distance between the 1 st point and the 3rd point was 5 mm,then the 2.5 mm step distance could be deemed to no deviation.The 5 mm step distance of this afterloading system had no deviation in continuous 9 step points measured by flushing-free film.The indirect measuring results of the 2.5 mm step distance had no deviation as well.The position accuracy of this afterloading system measured with the flushing-free film accorded with the national standards.Conclusions The method of measuring the position accuracy and the step distance accuracy of the afterloading system with192Ir source by using flushing-free film is technically feasible.
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Objective To scrutinize the positioning accuracy and reproducibility of the cone-beam computed tomography system in combination with the six-degree couch table (Hexapod Robot Treatment Table, HRTT). Methods The mechanical stability of the X-ray volume imaging (XVI) system was tested,in terms of the reproducibility. And the influence of the moveable parts, including the KV panel and the source arm, on the accuracy of the XVI image registration was analyzed. The accuracy between the bone and grey value registration was compared using a head-and-neck phantom. The accuracy of the HRTT for translational, rotational, and a combination of translational and rotational corrections was investigated in consecutive measurements. Results The performance of XVI system itself was stable with translational and rotational error of below 0. 4 mm and below 0. 3°, respectively. The mean position accuracy of the XVI system in combination with the HRTT summarized over all measurements was below 0. 6 mm and below 0. 4° for translational and rotational corrections, respectively. The grey value match was more accurate than the bone match. Conclusions The XVI image acquisition and registration procedure were highly reproducible.Both translational and rotational positioning errors can be corrected very precisely with the HRTT. The HRTT is therefore well suited to complement CBCT to take full advantage of position correction in six degrees of freedom for image guided radiotherapy.
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Objective To explorer the application in detection the position accuracy of the multileaf collimator of Varian accelerator with dynamic therapy log files. Methods A pre-designed MLC format files named PMLC for two Varian accelerators, the dynamic treatment log files were recorded 10 times on a different date, and be converted into the MLC format files named DMLC, compared with the original plan PMLC, so we can analysis two files for each leaf position deviation. In addition, we analysis the repeatability of MLC leaves position accuracy between 10 dynalog files of two accelerators. Results No statistically significant difference between the average position of the 10 times leaf position of the two accelerators,their were 0. 29 -0. 29 and 0. 29 -0. 30(z = -0. 77, P=0. 442). About 40% ,30% ,20% and 10% of the leaf position deviation was at ≤0. 2 mm, 0. 3 mm,0. 5 mm and 0. 4 mm,respectively. the maximum value was 0. 5 mm. More than 86% of the leaf position are completely coincident between 10 dynamic treatment files of two accelerators,The rate of position deviation no more 0. 05 mm was 96. 6% and 97.3%, respectively.And the maximum value was 0. 09 mm. Conclusions Dynamic treatment log file is a splendid tool in testing the actual position of multi-leaf collimator. The multi-leaf collimator of two accelerators be detected are precise and stabilized.
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Objective To design a new method to verify the position of multileaf collimator(MLC)leaf using a two-dimensional ion chamber array(2D-array).Methods 2D-array of PTW T10018 Seven29~(TM) was used to calibrate the accuracy of MLC leaf position of Elekta Precise accelerator.The edge function of the leaf position of MLC was measured and used as the reference value.The precision of MLC leaf was then evaluated through comparing the measured and reference values.Results The accuracy of MLC leaf position was found within?0.1 mm.Conclusion This method of verifying the accuracy of multileaf collimator leaf position is easy,simple and reliable