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Objective:To investigate the performance of W2 plastic scintillator in megavolt photon and electron beams.Methods:The photon and electron beam energy provided by linear accelerator was used to collect data of the W2 scintillator. The parameters include the electrometer reading stability, W2 dose and dose rate linearity, and angular response. And the dose uncertainty of the W2 correction factors was also investigated.Results:The standard deviation of the electrometer reading stability was between 0.03 and 0.47. The linear regression factors of W2 dose were all 1.0; the maximum deviation of the dose rates was 0.61%. The Cerenkov light radiation correction factor(CLR) for 6 and 10 MV were 0.741 and 0.746, respectively, and the CLR for 6, 9, 12 and 15 MeV were 0.750, 0.753, 0.757 and 0.757, respectively. The maximum deviation of dose uncertainty for 15 MeV was 3.15%.Conclusions:The signal obtained by the blue and green channel was no angular dependence, the same as the high-energy electron beam, which verified that the Cerenkov radiation correction factor has good linearity. W2 plastic scintillator can be applied to non-coplanar radiotherapy dosimetry.
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Objective:To solve the problems in intensity-modulated radiation therapy (IMRT) planning, such as large labor cost and high dependence on the experience of physicists and great inconsistency in the quality of plan, and to discuss an unsupervised automatic treatment planning procedure of IMRT.Methods:The eclipse scripting application programming interface (ESAPI) within the Eclipse treatment planning system (TPS) 15.6 and optimization parameters tree search algorithm (OPTSA) were used to emulate and realize the whole planning process. Interacted with the TPS through ESAPI, relevant dosimetric parameters were input and output. The OPTSA evaluated the plan qualities based on dosimetric parameters of the targets and organs at risk (OARs) and iteratively adjusted the optimization objective parameters to achieve a progressively improving IMRT plan. In order to verify the effectiveness of the automatic planning, twenty historical rectum cancer cases were selected from the clinical database, and the dose distribution and specific dosimetric parameters were compared between the plans generated by the OPTSA and the manual plans under the same constraints.Results:All the auto plans have met clinical requirements. Furthermore, 90% and 10% of the auto plans were deemed as clinically improved and equally compared with the manual plans, respectively. The average CI for the PTV was 0.88 and 0.80 for the auto and manual plans respectively. Compared with the manual plans, the mean doses of all the OARs in the auto plans were reduced by 11% in average. The average elapsed time of automatic planning and manual planning was (28.15±3.61) and (36.7±4.6) min, respectively.Conclusions:The plans created by the proposed algorithm have been shown to be at least as good as the manual plans. In addition, this method can shorten the labor time in plan designing while ensuring the plan quality and consistency of the plan.
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OBJECTIVE@#To explore the optimization scheme of maintaining bus voltage stability during turbo-turbine acceleration and deceleration of ventilator.@*METHODS@#The ideal diode is used to replace the diode in the busbar power supply circuit, and a comparative discharge circuit is added to the busbar. When the busbar voltage is higher than the preset threshold, the comparator can be opened and the energy could be discharged through the power resistor.@*RESULTS@#When the turbine starts and stops rapidly, the optimized scheme can effectively reduce the bus impedance, and the discharge circuit can maintain the bus voltage fluctuation less than 2 V.@*CONCLUSIONS@#The optimization scheme proposed in this study can effectively improve the efficiency and stability of the turbine in the process of acceleration and braking, and provide reference for the design of the stability maintenance circuit of the ventilator turbine bus.
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Electric Power Supplies , Ventilators, MechanicalABSTRACT
Objective:To compare and analyze the image quality, signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of T 1WI_Star_VIBE_FS sequence in MRI simulation of neck tumors with different scanning methods, aiming to determine the optimal scanning method. Methods:A retrospective analysis of 78 patients receiving MRI contrast scan was performed. All patients were randomly divided into three groups according to three different scanning methods including bolus mode (group A, n=23), segmentation splicing mode (group B, n=18) and the combination mode (group C, n=37). The image quality, SNR and CNR of the anterior soft tissues of neck were statistically compared. Results:A higher image quality score was obtained in group C. The mean SNR and CNR in three groups were calculated as 214.70±148.78, 91.95±59.26, 307.61±127.80, and 208.74±148.27, 85.79±59.50, 301.58±127.48, respectively. The image quality score, SNR and CNR in group C were significantly better compared with those in group A and B (all P<0.01). Conclusion:Combination of bolus and segmentation splicing modes is a recommended approach in MRI simulation during radiotherapy of neck tumors.
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Objective:To develope an automatic volumetric modulated arc therapy (VMAT) planning for rectal cancer based on a dose-prediction model for organs at risk(OARs) and an iterative optimization algorithm for objective parameter optimization.Methods:Totally 165 VMAT plans of rectal cancer patients treated in Peking University Cancer Hospital & Institute from June 2018 to January 2021 were selected to establish automatic VMAT planning. Among them, 145 cases were used for training the deep-learning model and 20 for evaluating the feasibility of the model by comparing the automatic planning with manual plans. The deep learning model was used to predict the essential dose-volume histogram (DVH) index as initial objective parameters(IOPs) and the iterative optimization algorithm can automatically modify the objective parameters according to the result of protocol-based automatic iterative optimization(PBAIO). With the predicted IOPs, the automatic planning model based on the iterative optimization algorithm was achieved using a program mable interface.Results:The IOPs of OARs of 20 cases were effectively predicted using the deep learning model, with no significantly statistical difference in the conformity index(CI) for planning target volume(PTV)and planning gross tumor volume(PGTV)between automatic and manual plans( P>0.05). The homogeneity index (HI) of PGTV in automatic and manual plans was 0.06 and 0.05, respectively( t=-6.92, P< 0.05). Compared with manual plans, the automatic plans significantly decreased the V30 for urinary bladder by 2.7% and decreased the V20 for femoral head sand auxiliary structure(avoidance)by 8.37% and 15.95%, respectively ( t=5.65, 11.24, P< 0.05). Meanwhile, the average doses to bladder, femoral heads, and avoidance decreased by 1.91, 4.01, and 3.88 Gy, respectively( t=9.29, 2.80, 10.23, P< 0.05) using the automatic plans. The time of automatic VMAT planning was (71.49±25.48)min in 20 cases. Conclusions:The proposed automatic planning based on dose prediction and an iterative optimization algorithm is feasible and has great potential for sparing OARs and improving the utilization rate of clinical resources.
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Objective:To characterize the key dose response properties of the novel presage sheet dosimeters for radiotherapy dose verification, including absorption spectra, linearity, dose range and stability.Methods:The same batch of presage sheet dosimeters were irradiated by a radiotherapy linac. The absorption spectra within 400-700 nm were read out with a spectrophotometer, and the R-G-B3 absorption changes read out with a film flatbed scanner was compared before and after irradiation.Results:An absorption peak was clearly identified at 628 nm, where absorptions change in high linearity with delivered doses ( R2=0.9999). A flat valley region is identified around 490 nm, where dose induced absorption changes were negligible. The readout sensitivity of the R-channel of the flatbed scanner was higher than both in green and blue channels. In the dose range below 10 Gy, the R-channel absorptions are in significant linearity with doses ( R2=0.9999), with absorptions change in an obvious quadratic trend in the range beyond 10 Gy ( R2=0.9999). The dose range of presage sheets was more than 94.6 Gy. The absorptions were well preserved within 1 h post-irradation, and then are shown to increase gradually, where the increase speeds are dose-related. The post-irradiation integrity of dose falloff gradients are shown with negligible gradient blurring. Conclusions:The novel presage sheets shown to have reasonable dose response linearity, large dose range, desirable post-irradiation dose gradient integrity and negligible fractionation effect, which indicates its great potentials in integral dose verification of high-dose and multiple target radiotherapy deliveries.
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Objective:To build patient setup workflow based on the optical surface monitoring system (OSMS) in postoperative radiotherapy for breast cancer, and compare the setup accuracy and PTV margin between OSMS based setup and conventional skin marker based method.Methods:The setup data of 20 cases of postoperative radiotherapy for breast cancer were retrospectively analyzed and divided into two groups: OSMS setup group and conventional skin marker setup group with 10 patients in each group. All CBCT rigid registration values in six dimensions ( x, y, z, Rtn, Pitch, Roll) were obtained, and the absolute value and distribution of errors were statistically analyzed by single sample t-test and χ2-test respectively. Finally, the CTV-PTV margins were compared using the extension formula. Results:The mean values in OSMS setup group and conventional skin marker setup group in sixdirections were 0.18 and 0.18 cm, 0.12 and 0.13 cm, 0.13 and 0.23 cm, 0.55° and 0.74°, 0.63° and 0.99°, 0.67° and 0.68°, respectively, while the standard deviations were 0.13 and 0.12 cm, 0.09 and 0.09 cm, 0.11 and 0.16 cm, 0.37° and 0.55°, 0.53° and 0.65°, 0.42° and 0.55°, respectively. The setup error differed in both z and Pitch directions( t=3.53, 2.98, P<0.05), while the error distribution rate difference was statistically significant between two groups in z direction( χ2=11.090, P<0.05). The CTV-PTV margins in OSMS setup group and conventional skin marker setup group were 0.28 and 0.26 cm, 0.21 and 0.20 cm, 0.24 and 0.35 cm, respectively. Conclusions:The proposed OSMS-based patient setup work flow is better than the conventional skin marker based method in setup accuracy, with significant setup error differences in z and Pitch directions. The proposed OSMS workflow is of potential clinical benefit.
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Objective:To develop a new method to set up patients using optical surface monitoring system and to compare it with the conventional method in head radiotherapy.Methods:A total of 358 set-ups (130 with the conventional method and 228 with the new method ), which were from 99 head tumor patients in Beijing Cancer Hospital treated between May 2018 to April 2019, obtained by using Image Guided Radiotherapy were retrospectively analyzed. The distributions of set-up errors, the number of abnormal positions, and the set-up time were compared to evaluate the potential advantages of the new method .Results:The mean (± standard deviation) absolute values of setup errors of the new method were (0.07±0.07) , (0.08±0.06) and (0.06±0.06) cm for the vertical, lateral, and longitudinal, (0.53±0.41)°, (0.59±0.44)° and (0.59±0.46)° for the rotation, pitch and roll, respectively. In the new method , the setup accuracy was improved( t=3.24-6.10, P<0.001)and the number of abnormal positions was greatly reduced(χ 2=60.66, P<0.001). Compared with the conventional method, the patient setup time was slightly reduced by the new method , but the difference was not statistically significant ( P>0.05). Conclusions:The new high-precision method to set up patients using optical surface monitoring system improves the accuracy of patients′ position, decreases the corrections applied by 6DoF couch, reduces the probability of abnormal positions, and suggests the potential benefit in head radiotherapy.
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Objective@#To compare the setup accuracy between Catalyst HD and skin markers in stereotactic body radiotherapy (SBRT) of lung cancer.@*Methods@#A total of 24 cases treated with SBRT were selected and all patients were fixed with vacuum pad in the supine position. Patients in group A were positioned by Catalyst HD and those in group B were positioned by shin markers. All patients were matched with the CT images after CBCT scan by rigid registration and the setup errors in six directions (x-, y-, z-axis, Rtn, Pitch and Roll) were obtained.@*Results@#The mean±SD in group A and B in the six directions were as follows: (0.13±0.12) cm, (0.25± 0.19) cm; (0.26±0.15) cm, (0.13±0.11) cm; (0.23±0.19) cm, (0.35±0.29) cm; (0.43°±0.40°), (0.80°±0.69°); (0.48°±0.47°), (0.79°±0.64°); (0.62°±0.60°) and (0.88°±0.70°), respectively. Except the x-axis data in group B, all the data in the six directions were not normally distributed. The obtained data significantly differed between two groups (all P<0.05). The out-of-tolerance errors (>0.5 cm/2°) also significantly differed between two groups (P<0.05).@*Conclusions@#The setup errors of Catalyst HD are less than those of the skin markers (except the y-axis). The setup accuracy of Catalyst HD is superior to that of traditional skin markers, which is worthy of application in clinical practice.
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Objective To compare the setup accuracy between Catalyst HD and skin markers in stereotactic body radiotherapy (SBRT) of lung cancer.Methods A total of 24 cases treated with SBRT were selected and all patients were fixed with vacuum pad in the supine position.Patients in group A were positioned by Catalyst HD and those in group B were positioned by shin markers.All patients were matched with the CT images after CBCT scan by rigid registration and the setup errors in six directions (x-,y-,z-axis,Rtn,Pitch and Roll) were obtained.Results The mean ±SD in group A and B in the six directions were as follows:(0.13±0.12) cm,(0.25± 0.19) cm;(0.26±0.15) cm,(0.13±0.11) cm;(0.23±0.19) cm,(0.35±0.29) cm;(0.43°±0.40°),(0.80°±0.69°);(0.48°±0.47°),(0.79°±0.64°);(0.62°±0.60°) and (0.88°±0.70°),respectively.Except the x-axis data in group B,all the data in the six directions were not normally distributed.The obtained data significantly differed between two groups (all P＜0.05).The out-of-tolerance errors (＞0.5 cm/2°) also significantly differed between two groups (P＜0.05).Conclusions The setup errors of Catalyst HD are less than those of the skin markers (except the y-axis).The setup accuracy of Catalyst HD is superior to that of traditional skin markers,which is worthy of application in clinical practice.
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Dose verification, as part of the radiotherapy QA chain, is a significant method to ensure the patients' safety and efficacy of treatment. The increasing application of precision radiotherapy techniques in clinic has advanced the need of three-dimensional ( 3D) dose verification. Gel dosimeters, prevailing for its intrinsic 3D high-resolution measurement and good tissue equivalence, can serve as effective supplement to the clinical radiotherapy dosimetric system. This paper reviews the method ology, dose response mechanism, characterizations of the state-of-the-art gel dosimeters. Gel dosimeters, outstanding for 3D dose measurement, have a great potential to explore both for clinical application and academic research.
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Objective To compare two pretreatment plan QA methods for HalcyonTM accelerator using Portal Dosimetry (PD),and PTW OCTAVIUS 1500 detector array + Octagonal phantom (Oct 1500)respectively.Methods Parallel measurement-based pretreatment QA using two methods was performed for 22 IMRT/VMAT plans (74 fields) that have been used to treat 20 patients recruited in the Halcyon clinical trial.Several γ 2D comparisons were also applied to provide guidelines for Halcyon planning QA.Results Using Oct1500 method,the γ 2D passing rates for 74 fields in 22 Plans were 95.26±3.59,95.01±3.62 (Local Dose),99.05± 1.35,98.57± 1.96 (Max Dose) respectively.Two-related samples non-parametric tests suggested that the differences between the evaluation criteria were of statistical significance (Z =-7.220,-4.108,P＜0.05).For PD method,the γ 2D passing rates were 84.11％ ± 1.35％ (1 mm/1％),99.07％± 1.35％ (2 mm/2％),and 99.86％ ± 1.35％ (3 mm/3％).Two-related samples non-parametric tests suggested that the differences between evaluation criteria of PD method were statistically significant (Z =-7.475,-7.475,-6.906,P＜0.05).For 74 fields and max dose,3 mm/3％ evaluation criteria,two-related samples non-parametric tests suggested that the differences between PD and Oct1500 method were statistically significant (Z=-5.072,P＜0.05).Conclusions Both methods can be used for Halcyon pretreatment plan QA.PD is superior to Oct1500 with respect to efficiency and spatial resolution-induced verification accuracy.The criteria of 2 mm/2％ for PD,and Max Dose/3 mm/3％ for Oct1500 was suggested.
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Objective To compare the impact of immobilization base plates composed of 7 types of materials on the MR-simulation imaging quality used for radiotherapy,aiming to provide reference data for clinical applications.Methods Using identical T1 and T2 sequences of Siemens MR-simulator,the MR images of ACR Large Phantom were acquired on the Orfit carbon fiber laminate,polycarbonate (PC),high precision base plate (HP),Jinan Huayuxin BR,WR,KP and SP materials,respectively.The imaging quality without any plate was used as the baseline data.The following metrics were compared:1.High-contrast spatial resolution:the sharpness of 3 pairs of hole arrays was observed,which represented resolutions of 1.1 mm,1.0 mm and 0.9 mm on the LR and AP directions;2.Image intensity uniformity in terms of percent integral uniformity (PIU):PIU =100× [1-((high-low)/(high +low)];3.Low-contrast resolution:distinguishable spokes representing resolutions of 5.1％,3.6％,2.5％ and 1.4％ were recorded.According to ACR recommendations,high-contrast resolution of 1.0 mm,PIU of T1WI and T2＞82％and low-contrast spoke difference＜3 were considered as clinically acceptable.Results High-contrast T1WI and T2WI resolutions of no plate,PC plate,HP plate and KP material were all 0.9 mm,and those of the remaining materials were =worse than 0.9 mm.The T1WI and T2WI PIU of no plate was＞87％,and the PIU of carbon fiber plate was reduced by＞ 25％.The PIU decrease of remaining materials was within ±4％.Except the carbon fiber plate,the low-contrast spokes of the remaining materials were within ±3 compared with the reference.Conclusions In clinical settings,carbon fiber plate exerts the most significant effect upon the MR-simulation imaging quality,which is unsuitable for MR simulation.Imaging quality of PC and HP plates are consistent with that of no plate.BR and KP materials exert slight effect upon MR signal.The remaining materials are of potential values for the manufacture of immobilization devices and accessories.
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Objective To explore the Monte Carlo calculation methods for the absolute dose calibration and output factor of 6 MV flattening-filter ( FF) and flattening-filter free ( FFF) photon beams based on TrueBeam accelerator .Methods The BEAMnrc code was used to model the LINAC head of FF and FFF modes.The BEAM_up covers the components from the target to the monitor chamber , and BEAM_down includes the structures beneath the chamber , the dose deposit to the monitor chamber contributed by the incidence electrons and scattered particles from the secondary collimators were calculated respectively .The incidence electron-induced dose at certain depths on the central axis were simulated by means of the DOSXYZnrc code .By means of dose calibration equation , the calibration factor for the standard field (10 cm ×10 cm) and the output factors for various fields (1 cm ×1 cm-40 cm ×40 cm) were computed respectively .Results For the 6 MV FF and FFF beams under the standard 10 cm ×10 cm field, 1 MU equals to 7.747 ×1013 ±3.099 ×1011 and 3.248 ×1013 ±1.624 ×1011 electrons to the target , respectively , which deposited 21.53 and 35.01 cGy to the monitor chamber of the virtual accelerator respectively .The difference between the simulated and calculated output factors were 0.72%±1.4%and 0.56%±0.78%for FF and FFF , respectively .Conclusions The model generated and measured output factors agree well , indicating the good accuracy of the dose calculation by the model , which would provides basis for further clinical dosimetric studies .
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Objective To evaluate the feasibility and dosimetric features of a volume modulated arc therapy (VMAT)-configured model in knowledge-based optimization of fixed-field dynamic intensitymodulated radiotherapy (IMRT) plans based on the Varian RapidPlan system.Methods ① A dose-volume histogram prediction model was trained with 81 qualified preoperative VMAT plans for rectal cancer and then statistically verified.② For clinically approved 10 IMRT plans with the same dose prescription,the above model was used to automatically generate new optimization parameters and dynamic muhileaf collimator (MLC) sequences with field geometry and beam energy unchanged.③ In order to rule out the disparities between different versions,a single algorithm was used to calculate the absolute doses of the original and new plans.④ Statistical analyses were performed on dosimetric parameters after comparable target dose coverage was achieved in the two plans by appropriate normalization.Results On the basis of similar target dose homogeneity and coverage,RapidPlan significantly reduced the doses to the urinary bladder (D50％ by 9.01 Gy,P =0.000;Dmean by 8.08 Gy,P =0.005) and the femoral head (D50％ by 4.20 Gy,P =0.000;Dmean by 3.84 Gy,P=0.005) but significantly elevated the mean total number of monitor units (1211±99 vs.771±79,P=0.000) and the number of fields with multiple MLC carriage groups.The knowledge-based semi-automated optimization caused a significantly larger number of high-dose hotspots but a similar D2％ (52.54 vs.52.71 Gy,P=0.239).Conclusions The VMAT model can be used for the knowledge-based semi-automated optimization of IMRT plans to enhance the efficiency and OAR protection.However,the resulting high-dose hotspots need further manual intervention.
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Objective To investigate the dose distribution and radiation risk of Varian thoracic cone beam computed tomography (CBCT) with default parameters with reference to Monte Carlo simulation and International Commission on Radiological Protection (ICRP) report 110.Methods EGSnrc/BEAMnrc code was used to simulate the material, thickness, and geometry of the kV CBCT source (kVS) to establish the kVS model.A benchmarked MCSIM code was applied to calculate the dose distribution in the ICRP phantom after the scan with the standard thoracic parameters (110 kV, 20 mA, and 262 mAs), and the conversion coefficient of absolute dose was obtained in a spherical phantom following the TG-61 protocol.The results of Monte Carlo simulation were validated by PDD and Profile in a water phantom and the measurement of the absolute dose in the computed tomography dose index (CTDI) phantom and Alderson phantom.The models including BEIR VⅡ were used to evaluate the radiation risks.Results With reference to the criterion of 3%/1 mm, the uncertainties of PDD and Profile were less than 2%.The difference between the measured and calculated values was<2.9% in the CTDI phantom and ≤0.05 cGy in the Alderson phantom.In the ICRP110 phantom, the doses to the left lung, right lung, left breast, right breast, heart, thyroid, trachea, cancellous bone, and cortical bone were 1.28, 1.39, 1.74, 1.80, 1.46, 0.48, 0.88, 0.85, and 1.84 cGy, respectively.The relative risks of ischemic heart disease, breast cancer, lung cancer, thyroid cancer, and tracheal cancer in a standard scan were1.001 , 1.009, 1.019, 1.000, and 1.008, respectively. Conclusions The accumulated dose and long-term risks of CBCT during image-guided thoracic radiotherapy cannot be neglected and should be effectively controlled.
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Objective Knowledge?based radiotherapy ( KBRT ) can reduce the plan quality variability induced by different experiences between physicians and improve the quality of treatment plans. Methods The Varian Rapid Plan system was used to train a dose?volume histogram ( DVH) prediction model. The obtained model was preliminarily applied to semi?automatic design of the preoperative treatment plans for rectal cancer. Eighty high?quality volumetric modulated arc therapy plans were imported into the model training set of the Rapid Plan system. The structures of the plans were matched to the corresponding labels and codes as listed in the library. The training started after the verification of prescription. The residual plots,regression curves,geometric plots for organ at risk ( OAR) ,in?field DVH plots,and model training logs were examined. After removal of the mismatch, the original plans were assessed to rule out outliers and influential data points. More similar plans may be added for another round of training. Ten KBRT plans were designed using the final model and compared with the clinical plans. Results For the two major OARs,the femoral head and bladder,the average goodness of fit of the principal component were 0?999 415/1.0 and 0?999 963/1.0 for the DVH model,and 0?999 651/1.0 and 0?999 945/1.0 for geometry?based expected dose model,respectively. In all the plans, 11 had Cook ’ s distance values exceeding the tolerance and 4 had studentized residual values exceeding the tolerance. The outliers were all kept in the training set to generalize the scope of the model. The 10 KBRT plans had significantly improved homogeneity indices for PGTV and PTV than the original plans (P=0?00,0?04).The 10 KBRT plans also had significantly reduced D50% to the femoral head and bladder as well as significantly reduced mean doses to the bladder than the original plans (P=0?042,0?000,0?005). Conclusions In this study,the Rapid Plan system is used to train a KBRT model for design of preoperative radiotherapy plans for rectal cancer. The results of preliminary application meet the clinical requirements.
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Objective To compare the performances of four commercially available LINAC daily QA instruments.Methods The dosimetric stability of a LINAC including central axis output,flatness and symmetry were verified and fine-tuned using a 3-dimensional water phantom,dosimeters and ionization chambers.The baseline of the four instruments including LINA-C,QUICK-C,BEAM-C and QA3 were set thereafter.Daily measurements of LINAC were conducted with these instruments respectively and the results were compared.Arbitrary errors (CAX and SYM) beyond TG-142 tolerances were introduced to the LINAC to test the sensibilities of each instrument in detecting these changes.Results Relative to the baseline that were measured by the 3-dimensional water phantom and dosimeters,the results monitored by the four instruments were comparable.The maximum disparities of the CAX,FLAT,and SYM were 0.5％ (LINA-C),-0.45％ (QUICK-C),and 0.5％ (BEAM-C),respectively.All checkers detected the known errors successfully.Conclusions The stabilities of all the four evaluated instruments met the requirements of daily QA for LINAC.LINA-C verifies CAX only.QUICKE-C,BEAM-C and QA3 can be used to perform all the daily QA protocols as suggested by AAPM TG 142 report.They also provide unique additional functions.The setup of baseline determines if the morning checkers could measure the LINAC dosimetric parameters correctly.When an error is alarmed by the morning checker,it is recommended to verify the performance of the instrument first rather than recalibrating the LINAC immediately.
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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 compare the dosimetric difference of volumetric modulated arc therapy (VMAT) plans for the loco-regionally advanced nasopharyngeal carcinoma (NPC) patients using 6 MV X-ray flattening filter-free (FFF) and flattening filter (FF) modes.Methods Ten previously treated patients with loco-regionally advanced nasopharyngeal carcinoma were retrospectively included and replanned using FFF and FF VMAT,respectively.Meeting all clinical criteria,the dose-volume histograms (DVH),dose distribution of target volume and organs at risk (OARs),target conformity index (CI),total monitor unites (MUs) and treatment time were compared across the plans.Results VMAT plans suing either FFF and FF modes can meet the clinical objectives.The maximum and mean target dose of FFF VMAT plans were significantly higher than that of FF VMAT plans (t =-0.31,-O.35,P ＜ O.05).Yet the planning target volume (PTV) CI of FF mode was slightly better than of FFF mode (t =5.42,P ＜0.05).The maximum doses of lenses in FFF VMAT plans were lower than that of FF VMAT plans (t =25.87,17.45,P ＜ 0.05),and other OARs displayed no significant difference.The mean total MUs of FFF and FF VMAT plans were (699 ± 16) and (628 ± 12) MUs respectively.The mean treatment time of two modes were consistent (about 2 min).Conclusions The plan qualities of FFF and FF VMAT plans were comparable and both clinically acceptable.The OARs at the edge of radiation field,such as lens,could be spared better using FFF VMAT mode.The total MUs of FFF VMAT plans were higher than FF VMAT plans,yet were delivered within the same time.