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Objective:To study the influence of intensive magnet fields on radiation dose measurement, and to demonstrate the feasibility of measuring magnet field correction factor by a combination of medical linac with variable magnet fields in view of needing for accurate measurement of the doses from reference beam arising in MR image-guided radiotherapy.Methods:A photon radiation field and a variable field with 6 MV nominal high voltage were produced by using conventional medical electron linear accelerator equipped with a pair of electromagnets with magnetic field strength up to 1.5 T. Both PTW30013 and PTW31010 ionization chambers were used to test the responses of ionization chambers under different magnetic field strengths at four orientations in which the angles between ionization chamber axis and magnetic field direction were 0°, 180°, 90° and 270°, respectively. The magnetic factors, kB, M was calculated and compared with the reported values in literature. Results:The response of ionization chamber was proportional to the magnetic field strength before it reached to a peak around 1 T, and then fell down as the magnetic field continued to rise. When the magnetic field was 0.35 T, the magnetic factors of PTW31010 were 0.988 2±0.000 3 and 0.997 4±0.000 4 corresponding to 90° and 0° directions, the discrepancy between 0° scenario and literature was 0.05% ± 0.04%. When the magnetic field reached 1.5 T, the magnetic factor of PTW30013 was 0.958 9±0.000 5 at the situation of 90°, which was 0.60% ± 0.05% different from the literature value.Conclusions:Conventional 6 MV medical accelerator equipped with electromagnet can be used to measure the magnetic field factor of reference dosimetry for MRIgRT.
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Objective:To study the correction factor of the chamber wall of the reference air kerma rate reference ionization chamber (NIM-Ir-SG-100 type) for 192Ir radioactive source. Methods:The photon spectrum and ionization chamber wall correction factor of the radioactive source housing and irradiator model are calculated by Monte Carlo code. And then the photon energy, wall thickness and ionization chamber diameter, which affect the result of the chamber wall correction factor, are simulated.Results:After calculation, the simulation result of the correction factor of spherical graphite cavity ionization chamber wall was 1.037 7. Controlling a single variable, photon energy (0.3-1.3 MeV), wall thickness (0.2-0.5 cm), inner diameter (0.5 -15 cm), the maximum deviations of wall correction result were 1.62%, 3.30% and 2.86%, respectively.Conclusions:The self-made spherical graphite cavity ionization chamber has good performance, and the corrected kwall value of the chamber wall is within a reasonable range. The completion of the chamber wall correction factor is an important step to measure the reference air kerma rate of the 192Ir radioactive source and establish the measurement benchmark.
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Objective:To explore the feasibility of the clinical implementation of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans with 6MV photon on two Elekta Linacs (Versa HD and Synergy) after beam matching.Methods:The images of 12 patients with nasopharyngeal carcinoma, central lung cancer and prostate cancer were randomly selected, and the IMRT and VMAT plans were designed. Two different dose tools of ionization chamber and three-dimensional detector ArcCheck were used to verify the individualized radiation treatment of 6MV photon beams on two Linacs and compare the differences.Results:The deviations between the doses of two Linacs (Versa HD and Synergy) measured by the ion chamber and treatment planning system were (0.32±1.32)% and (0.54±1.29)%. The differences of all plans were within the range of ±3%, and the deviations of the point dose between two Linacs were within the range of ±2% with no statistical significance (both P>0.05). The γ analysis of verification using ArcCheck showed that the passing rates of all plans under the 2mm/3% and 3mm/3% with 10% threshold conditions were over 95%, respectively. The average differences between two Linacs were 0.19%(2mm/3%) and 0.09%(3mm/3%). Conclusions:The results of performing IMRT/VMAT plans on two Linacs meet the clinical requirements and the differences between two Linacs are small. Hence, the same plans can be implemented interchangeably on different Linacs.
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Objective:To investigate the application of therapeutic grade ionization chamber to rapid measurement of short pulsed and high-dose-rate X-ray.Methods:The half-value layer of pulsed X-ray caused by an electron accelerator was measured by interpolation method and its equivalent energy was estimated. The cumulative doses from a certain amount of pulsed radiation at different distances in the same direction around the equipment were compared using the therapeutic grade ionization chamber and thermoluminescence measurement method . The relationship between the measurement result by using ionization chamber dosimeter and the distances away from source was analyzed. The cumulative doses from a certain amount of pulsed radiation at the same location at different frequencies were compared.Results:In working condition, 100 pulses of radiation were received accumulatively at 1 to 12 meters away from the outer wall of the equipment. The range of air Kerma was 0.08-9.65 mGy measured by using thermoluminescence dometers and 0.08 - 9.85 mGy using the ionization chamber dosimeters, respectively. The difference between both is within 10%. At different frequencies (1-10 Hz), there was no significant difference in X-ray air Kerma from 100 pulses measured by ionization chamber dosimeter at 2 m away from the front of the equipment ( P>0.05). Conclusions:The therapeutic grade ionization chamber dosimeter can be used for the rapid measurement of short pulsed X-ray radiation dose in the range of dose rates and pulse frequencies involved in the experimental accelerator device.
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Objective To investigate the application value of a transmission reference stealth ionization chamber in the relative dosimetry of stereotactic radiotherapy.Methods The relative dosage of Varian (R) Novalis 6 MV photon rays equipped with Brainlab (R) conical applicators was determined by two different approaches in the IBA (R) Blue Phantom.One approach was designed to utilize IBA (R) SFD3G semiconductor ionization chamber alone in the "step by step" measurement mode,whereas the other method was to utilize IBA (R) SFD as the field detector combined with stealth ionization chamber as the reference detector in the "continuous" measurement mode.The central axial percentage depth dose (PDD) and off axis ratio (OAR) in a diameter of 4 and 15 mm were measured and the time spent on each measurement was recorded.Results The PDDs at the central axis and OARs at a depth of 10 cm for both conical applicators were in good agreement between the two measurement approaches with a dose difference of＜ 1％.The time spent on PDDs and OARs measurement with stealth ionization chamber was shortened by 13.1％ and 20.7％ compared with those without Stealth ionization chamber.Conclusion Stealth ionization chamber can be applied in the retative dosimetry of stereotactic radiotherapy with high dose accuracy and high efficiency.
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Objective Because of statistical noise in Monte Carlo dose calculations,the effective point doses may not be accurately calculated.A user-defined sphere volume was adopted to substitute the effective point to take sphere sampling around the effective point,which minimize the random errors and improve the accuracy of statistical dose.Methods Direct dose measurements were performed at 0°and 90° using a 0.125 cm3 Semiflex ionization chamber (IC) 31010 isocentrically placed in the center of a homogeneous Cylindric sliced RW3 phantom (PTW,Germany).In the scanned CT phantom series,the sensitive volume length of the IC (6.5 mm) was delineated and the isocenter was defined as the simulated effective point.All beams were simulated in the treatment planning system (TPS) in accordance to the measured model.The grid spacing was calculated by 2 mm voxels and the relative standard deviation should be ≤ 0.5％.The statistical and measured doses were statistically compared among three IC models with different electron densities (ED;esophageal lumen ED =0.210 g/cm3 for model A,air ED =0.001 g/cm3 for model B and the default CT scanned ED for model C) at different sampling sphere radius (2.5,2.0,1.5 and 1.0 mm) to evaluate the effect of Monte Carlo.calculation uncertainty upon the dose accuracy.Results In the Monaco TPS,the statistical value was in the highest accordance with the measured value with an absolute average deviation of 0.49％ when the IC was set as esophageal lumen ED =0.210 g/cm3 and the sampling sphere radius was 1.5 mm.When the IC was set as air ED=0.001 g/cm3 and default CT scanned ED,and,the recommended statistical sampling sphere radius was 2.5 mm,the absolute average deviations were 0.61％ and 0.70％.Conclusion In the Monaco TPS,the calculation model with an ED of 0.210 g/cm3 and a sampling radius of 1.5 mm is recommended for the ionization chamber 31010 to substitute the effective point dose measurement to decrease the random stochastic errors of Monte Carlo.
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Objective To investigate the effect of the angle correction factors of ionization chamber array Matrixx on the dosimetric verification of volumetric modulated arc therapy plan.Methods The Matrixx (IBA)was put in the middle of the MultiCube Phantom. Computed tomography scan was performed and the results were sent to the Raystation treatment planning system(TPS). All data calculated from TPS and actually measured using MatriXX were gained under the following conditions:6 MV,100 MU,28 cm×28 cm radiation field. And the directions of beam were from 0°to 180°with an interval of 5°(except an interval of 1° from 85° to 95°). The angle correction factors(CF)(θ)cor of MatriXX were calculated by the calculated dose values from TPS and the measured values using MatriXX and then mirrored to the opposite side. CF (θ)cor values were compared with CF(θ)def values that were given by manufacturer. The comparison was made by paired t-test. Results CF(θ)cor and CF(θ)def were different, and the difference was significant between 85° and 95°,with a maximum of 7.5%(P<0.05). The CF(θ)cor correction method had a higher gamma pass rate than the CF(θ)def correction method,with a maximum of 17%(P<0.05). Conclusions The ionization chamber array MatriXX has an angular dependence and the angle correction should be performed.The angle correction factors have the individual characteristics and are different in every unit ionization chamber of the MatriXX. Therefore, it is necessary to do angle correction for every unit ionization chamber of the MatriXX and the possible errors caused by various factors should be considered in order to improve the accuracy rate and gamma pass rate of the plan verification.
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Objective:To analyze and compare the dose monitoring and control systems of two types of medical linear accelerator systematically from the view of mechanical and electrical engineering, and establish the method of the quality assurance of the dosimetry system and the solution in the routine maintenance.Methods: To systematically compare and analyze the system of dose monitoring and control for two types of linear accelerators (Precise Elekta and C-series of Varian Medical Systems) from twofold aspects that included machine physics and electric engineering of medical linear accelerators.Results:The monitoring system that took ionization chamber as center was established between connotation of machine physics and concrete realization of electromechanical engineering, and explained the difference and similarity between open type and sealed type of ionization chamber, and the theory that Precise series needn't be serviced by PFN.Conclusion: Based on the analysis of ionization chambers structure of medical linear accelerators on the principle of quality assurance system, proposed dosimetric testing and adjustment method, and provide reference for the establishment of quality assurance system and technical procedures of the maintenance engineering.
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Objective To verify and adjust the leaf position accuracy of a multi-leaf collimator (MLC) in a linear accelerator.Methods Ionization chamber arrays ( StarTrack, IBA) were used to measure the leaf position accuracy of a MLC in a linear accelerator (Precise, ELEKTA) and adjust the leaves out of tolerance.Results After the adjustment of leaf position of the MLC, the analysis of the verification films taken at the offset position showed that the leaf position of the MLC was accurate.Conclusions The method developed in this study is convenient and simple for measurement and calibration of leaf position of the MLC in the linear accelerator, which meets the MLC position accuracy requirement for the linear accelerator.
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Objective To determine the peripheral dose ( PD) of a Trilogy accelerator under different conditions and the feasibility of PD measurement using the semiconductor diode ionization chamber. Methods In a solid water phantom, a CC13 air?filled ionization chamber and a semiconductor diode ionization chamber were used for PD measurements with different distances (13 measurement locations within 1?31 cm) , depth ( 3, 5, 15 cm) , field sizes ( 10, 20, 30 cm) , wedge ( W15, W45, VW15, VW45) , and beam energy (6, 18 MV). The relationship of PD with PDleakage and PDscat er was determined by removing the scatter phantom. Simulating the patients with cervical cancer undergoing radiotherapy, a CIRS phantom received volumetric modulated arc therapy ( VMAT) , step?shoot intensity?modulated radiotherapy ( IMRT) , and sliding?window IMRT to measure PDs of the breast, thyroid, and lens. All the data were normalized to the isocenter. Results PD was gradually reduced with the increase in distance ( 13?41% at 1 cm from the edge to 0?25% at 31 cm from the edge) . With a fixed distance from the edge of the radiation field, there was no significant difference in PD between different depths. A radiation field with a size of 30 cm had a PD about two?fold higher than that with a size of 10 cm. PD increased with the increase in the physical wedge angle and increased by 1% compared with the open field;PD decreased with the increase in the virtual wedge angle and decreased by 2?3% compared with the open field. PD decayed from 13?35% at 1 cm to 0?23% at 31 cm under 6 MV X?ray and from 11?06% at 1 cm to 0?20% at 31 cm under 18 MV X?ray. Dscat er was dominant in the regions close to the edge of radiation field and decreased from 62?45% at 1 cm to 5?71% at 25 cm. In all measurements under 6 MV X?ray, the maximum proportion difference between CC13 ionization chamber and diode ionization chamber was less than 1%. PDs of the breast, thyroid, and lens were 6?72, 2?90, and 2?37 mGy in VMAT mode, 7?39, 4?05, and 2?48 mGy in step?shoot IMRT mode, and 9?17, 4?61, and 3?21 mGy in sliding?window IMRT mode, respectively. Conclusions For the measurement of PDs, the CC13 air?filled ionization chamber and semiconductor diode ionization chamber have good consistency and feasibility under 6 MV X?ray. In clinical practice, the understanding of the relationship of PD with different radiation conditions helps to reduce the doses to organs at risk. Shielding and protective techniques can further reduce dose deposition.
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Objective To investigate the virtual wedge (VW) dosimetric parameters with the ionization chamber array Matrixxenvlution.Methods Using Matrixxenvlution and solid water to measure and calculate Siemens accelerator's VW angle and VW factors of different fields and compare the wedge field dose distribution to that of treatment planning system (TPS) by gamma analysis,summarized the measurement results of 50 times.Results The γ pass rate (3 mm/ 3％) of 15° and 30° VW in both direction were (91.47 ± 1.76)％,(92.99 ± 1.54)％ and (93.27 ± 1.24)％,(93.27 ± 1.68)％,respectively,with the increase of filed size and the VW angle,but for 20 cm ×20 cm field and VW 60°,the result was not very good.The largest angle deviation ＜ 2° except small field size and wedge angle,VW factors were approximately equal to 1,the maximum deviation was no more than 0.05,plan dose distribution and the measured dose distribution have good consistency except large field with large wedge angle.Conclusions Matrixxenvlution used in the measurement of VW dosimetric parameters which can obtain all parameters for angle calculation and dose plane analysis with only once positioning,and become more rapid,convenient,economical and practical one of quality assurance tools for VW dose verification.
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Objective To develop the method of analyzing α-radionuclides using large area grid ionization chamber.Methods Ultrasonic dispersion and vacuum drying system was used to prepare sample source,large standard thin source and plutonium plane source were used to optimize the working condition of spectrometer,and calibrate the instrument for analyzing α emitters.The certified reference materials (GBW04127) were used to verify the accuracy of the method.Results The non-linearity of calibration curve for standard thin sources of neptunium,plutonium and americium was less than 0.2％,and the resolution were 112,84 and 106 keV,respectively.The counting efficiency was 31.2％ for the large standard thin source.The values of specific activity measured in this way were in good agreement with those of the certified reference materials.232Th,238U,230Th,234U/226Ra,210Po,222Rn and 218Po were analyzed in a uranium mineral sample,and their specific radioactivity values were 5.3,3.8,35.6,21.4,27.0,19.6 and 11.1 Bq/g,respectively.Conclusions The method can be used to analyze α spectrum quickly in low-level radioactive environmental samples.
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Objective To verify the reliability of dose parameters of radiotherapy under reference and non-reference conditions by using TLD.Methods Dose parameters were verified by using TLDs under reference and non-reference conditions,including the maximum dose in axel of 5 electron beams with energy of 9 MeV and the variations of dose by depth,source-skin distance,exposure field and 45° wedge for 10 photon beams with energy of 6 MV in 5 hospitals.Results The average relative deviation of 6 MV photon beam measured between TLDs and finger ionization chambers were 4.45％,within ± 7％ as required by IAEA.The average relative deviation of 9 MeV electron beam measured between TLDs and plane parallel chambers were 2.45％,within ± 5％ was required by IAEA.Conclusions Measuring dosimetric parameters by using TLDs under reference and non-reference conditions was reliable and feasible.
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Objective To verify the methodology for auditing dosimetric parameters in reference and non-reference conditions with thermoluminescent dosimeters (TLDs).Methods Under reference and non-reference conditions,the established TLD methods were used to observe the absorbed dose variations with depth,SSD,field size and 45 wedges for 10 photon beams at 5 hospitals.Dosimetric parameters,including doses at Dmax points in axis,on 5 electron beams of 9 MeV were measured.The measurement results were compared between the TLDs and plane parallel ionization chambers.Results For 6 MV photon beams,the relative deviation of between finger ionization chamber method and TLD chips was in the range of-1.7％ to 5.4％ under on-axis non-reference conditions,and-6.3％ to-0.6％ under off-axis non-reference conditions,respectively,all within the range of ≤ ± 7％ as required by the IAEA.The relative deviation between plane parallel chamber and TLD method was-2.3％ to 3.7％,within ± 5％ as required by the IAEA.Conclusions It is convenient and feasible to use TLD method for quality audits of dosimetric parameters in radiotherapy.
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Objective To verify the reliability of TLD-based quality audit for radiotherapy dosimetry of medical electron accelerator in non-reference condition by monitoring the dose variations from electron beams with different field sizes and 45° wedge and the dose variations from photon beams with different field sizes and source-skin distance.Methods Both TLDs and finger ionization chambers were placed at a depth of 10 cm in water to measure the absorbed dose from photon beams,and also placed at the depth of maximum dose from electron beams under non-reference condition.TLDs were then mailed to National Institute for Radiological Protection,China CDC for further measurement.Results Among the 70 measuring points for photon beams,58 points showed the results with a relative error less than ± 7.0％ (IAEA's acceptable deviation:± 7.0％) between TLDs and finger ionization chambers measurements,and the percentage of qualified point numbers was 82.8％.After corrected by P,value,62 points were qualified and the percentage was up to 88.6％.All of the measuring points for electron beams,with the total number of 24,presented a relative error within ± 5.0％ (IAEA's acceptable deviation:± 5.0％) between TLDs and finger ioization cylindrical chambers measurements.Conclusions TLD-based quality audit is convenient for determining radiotherapy dosimetric parameters of electron beams in non-reference condition and can improve the accuracy of the measuring parameters in connection with finger chambers.For electron beams of 5 MeV ＜ E0 ＜ 10 MeV,the absorbed dose parameters measured by finger ionization chambers,combined with TLD audit,can help obtain the precise and reliable results.
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Objective To verify the reliability of radiotherapy dosimetric parameters in reference and non-reference conditions using thermoluminescent dosimeters (TLDs).Methods Using the established TLD method,the dose variations with different radiation field sizes and 45 ° wedge plate were verified for 10 photon beams of 6 MV,together with dosimetric parameters at the point of maximum axial dose for 4 electron beams of 9 MeV under reference and non-reference conditions.Comparisons were made between TLD results and finger ionization chamber results.Results The average relative deviation,for 6 MV photon beams,between TLD results and finger ionization chamber measurements was 4.7％,within ± 7％ as required by the IAEA.The average relative deviation,for 9 MeV electron beam,between TLD results and plane parallel ionization chamber measurements was 2.4 ％,not beyond ± 5％ permitted by IAEA.Conclusions Using TLD method to verify the radiotherapy dosimetric parameters in reference and non-reference conditions was reliable,simple and feasible.
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Objective To explore the MatriXX measurements the dose distributions for each beam in intensity-modulated radiotherapy (IMRT) plans were measured with 0 degree gantry angle and actual gantry position respectively.To discuss whether the two multi-angle synthetic pass rate from the two methods has statistics differences.Methods The dose distributions for each beam in IMRT plans were measured with 0 degree gantry angle and actual gantry position for twelve patients with head and neck tumor respectively.The γ pass rates (according to 3％/3 mm) for each beam under each angle condition was obtained by the comparison between the measured and the calculated dose distributions from the treatment planning system which was treated as the reference distribution.Use the t-test to analyse the actual gantry angle method and use the one factor analysis of variance to analyze the two multi-angle synthetic pass rate from the two methods.Results The γ pass rates of actual gantry angle was found generally declined seemingly compared with 0 degree gantry angle,but differentγ pass rates showed only in 80 °,120°and 240° with98.71％,93.59％(t=2.10,P=0.000),98.15％,93.17％ (t=2.10,P=0.000) and 98.94％,92.85％ (t =2.10,P =0.000) respectively.The γpass rate of multi-angle synthetic was seemingly between methods (98.27％,94.63 ％,F =0.50,P =0.134).Conclusions Two kinds of IMRT verification mode are from two position to validated the IMRT plans dose accuracy,comparatively analysing the conclusions drawn from the two methods can protect accuracy of IMRT plans more comprehensively.
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Objective To study the difference in the measurement of the absorbed dose in water between the dosimetry protocols of IAEA TRS-398 and TRS-277 for high-energy electron beams.Methods The differences were compare in absorbed doses from 6 kinds of electron beams among the three methods of using cylindrical chamber,plane-parallel chamber and cross-calibrated plane-parallel chamber which was calibrated in user' s high-energy electron beam according to IAEA TRS-398 and IAEA TRS-277.Results The difference in absorbed doses measured,following the two protocols,was 0.4％-2.3％ for planeparallel chamber,0.6％-2.2％ for cylindrical chamber,and 0.5％-2.0％ for cross-calibrated chamber.The differences in measured absorbed doses between the two dosimetry protocols were slight.Conclusions The methods used to determine absorbed dose to water recommanded by IAEA TRS-398 for high-energy beams are more accurate and more suitable for clinical users to measure compared to the TRS-277.
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Objective The beam data is compared with those obtained from Monte Carlo (MC)simulation and measurement to investigate their feasibility and reliability for X-ray small fields.MethodsThe beam data,including the total scatter factor (Scp),percentage depth dose (PDD) was acquired byneasurement and calculation with the field size ranging from 0.5 cm × 0.5 cm to 10 cm x 10 cm.The resultswere compared and analyzed.Results All the data is most consistent for the fields size of ≥3.5 cmx 3.5cm,but they are obvious different for the fields size of ≤ 3.0 cm × 3.0 cm.The measurements seem toreliable using the chambers of CC04 and CC13 for the fields size of ≥2.0 cm x 2.0 cm.Conclusions It isdemonstrated that the accurate measurements and calculations of Scp and PDD can be obtained for the fieldssize of ≥2.0 cm ×2.0 cm,but they needed morc rcscarchcs for thc smaller fields.
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Objective To facilitate activity measurement by using the thimble ionization chamber in hospitals,to obtain air kerma scatter correction factor of medical afterloading of 192Ir source by developing an available and convenient calculation method.Methods According to International Atomic Energy Agency (IAEA) 1079 Report to calculate the scatter correction factor of 192 Ir source,to measure air kenna of 192Ir source with and without lead shield using thimble ionization chamber.Simulation measurement conditions were used to calculate scatter correction factor of 192Ir source and comparison was made between experimental results and literature records.At the same time,the different ionization chamber models were simulated at different room sizes to obtain scattering correction factor of 192 Ir source.ResultsComparison was made between the simulation scatter correction factors of 192Ir source and experiment by the shadow shield,and the relative deviation was 0.8%.The deviation of the 192 Ir activity calculated according to the simulated scatter correction factor and measured by well type ionization chamber was 2.4%.By comparison between the calculated results by using two kinds of spherical ionization chamber and those ones deduced by IAEA 1079 Report,the relative deviations ranged within 0.3%-0.4%.Five different types of thimble ionization chamber and different room sizes were simulated and calculated by MC simulation,with the relative deviation within 3%.Conclusions Monte Carlo simulation method for calculating afterloading 192 Ir source's scatter correction factor is feasible,and this method is convenient for use in the thimble chamber for brachytherapy QA work in the hospital.