后装治疗源192 Ir空气比释动能强度检测 与评价

目的 研究用井型电离室测量后装192Ir源空气比释动能强度的方法.方法 用CDX-2000A静电计和HDR 1000井型电离室,现场检测30台后装192Ir源空气比释动能强度,根据源外观活度与空气比释动能强度转换系数,计算源外观活度.用实测源活度与厂家给出的初始源活度比较,相对偏差应在±5%内符合要求.结果 对所有检测的30台后装192Ir源活度与厂家初始源活度比较,相对偏差在-0.1%～4.4%范围内.结论 井型电离室测量法简便,准确度高,在医院可用于质量控制检测.

Abstract：

Objective To study the method of measuring air kerma strength of afterloading units with 192Ir source by using well type ionization chamber.MethodsThe air kerma strength of 30 afterloading units with 192Ir source was measured using 2000A electrometer and 1000 plus well type ionization chamber,and apparent activity of the source was calculated with the air kerma strength and apparent activity conversion factor.The measured activity of the source was compared with the original value of the source provided by the manufacturer,and the relevant deviation should be within ± 5%.Results Theair kerma strength of afterloding units with 192Ir sources was tested.The relevant deviation of the measured activity and the original value was within -0.1%-4.4%.Conclusions The measurement method with a well type ionization chamber is convenient and highly accurate which can be used for the test of quality control in hospitals.     

Low-dose-rate iodine-125 seed air kerma strength measurement intercomparison

PurposeThe purpose of this study was to investigate the rate of compliance of air kerma strength (AKS) measurements of iodine-125 (I-125) seeds with international recommendations by departments in Australia and determine the potential impact of noncompliance.Methods and MaterialsTo achieve this aim, we present an intercomparison of AKS measurements for a single I-125 seed performed by 11 radiotherapy departments in Australia. Measurements were performed at two sites, with each participating department traveling to one of the two host sites and measuring the AKS using their own equipment and local protocols. Each of the AKS measurements was compared with each other and the manufacturer-certified AKS.ResultsNine of the 11 participating departments measured AKS fell within ±3% of the manufacturer's calibration certificate value, whereas all participating departments measured AKS within ±5% of the manufacturer's calibration certificate value. The total spread of the measured AKS among the 11 departments was 7.7%. Only two of the 11 participating departments complied with international recommendations and had their well chamber calibrated within the last 2 years. In addition, 2 of the 11 departments used a well chamber calibrated that was calibrated with a different seed model used during the intercomparison, whereas 4 of the 11 departments calibrated their well chamber “in-house” using a factory-calibrated seed provided by the seed manufacturer.ConclusionsA significant variation in the methods used and frequency of calibration of well chambers were observed among the participating departments. The results of this study support the international recommendations on frequency and methodology of well chamber calibration. Failure to follow these recommendations significantly increases the uncertainty in AKS measurement of I-125 seeds.     

Dosimetric consideration of individual 125I source strength measurement and a large-scale comparison of that measured with a nominal value in permanent prostate implant brachytherapy

Purpose We investigated the difference between measured and manufacturer's nominal source strength in a large sample of a single model of ¹²⁵I seeds. Physical characteristics of single seed measurement by the well-type ionization chamber were also investigated to provide dosimetric data. Materials and methods A well-type ionization chamber with a single seed holder was used to measure source strength of all 1935 ¹²⁵I seeds implanted in the initial 28 patients in our hospital. Physical characteristics including linearity of readings for different integral time intervals, reproducibility, isotropy, and axial positional sensitivity were assessed. To calculate the source strength, the integral charge during 30 s was measured and converted to air kerma strength. The nominal activity stated by the manufacturer was compared with the measured value. Results Linearity, reproducibility, and isotropy of the well-type ionization chamber were within 0.2%. Measured source strength was on average 2.1% (range −7.6% to +7.2%), lower than the nominal value. Standard deviation of all measured seeds was 2.0%. The maximum difference between the measured and the manufacturer's nominal source strength in each patient was −3.7%. The standard deviation averaged 1.6%. Conclusion The nominal source strength of the ¹²⁵I seeds agreed well with the measured value. Our study can be helpful as guidance for individual ¹²⁵I seed source strength measurement.     

Determination of an air kerma-rate correction factor for the S7600 Xoft AxxentⓇ source model

PurposeThe purpose of this work was to provide guidance for the lack of an air-kerma rate standard for the S7600 Xoft Axxent® source by providing a correction factor to apply to the National Institute of Standards and Technology (NIST) traceable S7500 well chamber (WC) calibration coefficient before the development of an S7600 standard at NIST.METHODS AND MATERIALSThe Attix free air chamber (FAC) at the University of Wisconsin Medical Radiation Research Center was used to measure the air-kerma rate at 50 cm for six S7500 and six S7600 sources. These same sources were then measured using five standard imaging HDR1000+ WCs. The measurements made with the FAC were used to calculate source-specific WC calibration coefficients for the S7500 and S7600 source. These results were compared to the NIST traceable calibration coefficients for the S7500 source. The average results for each WC were then averaged together, and a ratio of the S7600 to S7500 WC calibration coefficients was determined.ResultsThe average S7600 air-kerma rate measurement with the FAC was 7% lower than the average air-kerma rate measurements of the S7500 source. On average, the S7500 determined WC calibration coefficients agreed within ±1% of the NIST traceable S7500 values. The S7600 WC calibration coefficients were up to 16% less than the NIST traceable S7500 values. The final correction factor determined to be applied to the NIST traceable S7500 value was 0.8415 with an associated uncertainty of ±8.1% at k = 2.ConclusionsThis work provides a suggested correction factor for the S7600 Xoft Axxent source such that the sources can be accurately implemented in the clinical setting.     

192Ir γ射线球形石墨空腔电离室室壁修正系数的模拟计算

目的 研究¹⁹²Ir放射源参考空气比释动能率基准电离室(NIM-Ir-SG-100型)的室壁修正系数。方法 利用蒙特卡罗程序计算经过放射源包壳和辐照器模型的光子光谱和电离室室壁修正系数,并对影响室壁修正系数结果的光子能量、壁厚和电离室内径进行了模拟。结果 经计算,球形石墨空腔电离室室壁修正系数模拟结果为1.037 7。控制单一变量,光子能量(0.3~1.3) MeV、壁厚(0.2~0.5) cm、电离室内径(0.5~15) cm对室壁修正系数结果的最大偏差分别为1.62%、3.30%、2.86%。结论 自制球形石墨空腔电离室性能良好,室壁修正系数k_wall值在合理范围内。室壁修正系数的完成为测量¹⁹²Ir放射源的参考空气比释动能率,建立计量基准完成重要的一步。     

测量近距离治疗源活度的井型电离室研制

目的研制井型电离室,测量近距离源空气比释动能强度,与国际标准测量物理量接轨,改善源活度的测量精确度,促进国内近距离剂量学的发展。方法吸取国外先进经验,结合本国国情,设计方案,绘制图纸,对井型电离室所有材料进行筛选,加工,组装,并开展性能实验。结果用国外进口的井型电离室和研制的井型电离室在相同条件下比对,结果:井型电离室长期稳定性为0.4%,技术指标为2%;电离电荷复合率为0.9995,技术指标0.9996;井型电离室重复性为0.02%,技术指标为0.5%。井型电离室的最佳驻留位置在平坦峰值区范围内灵敏度固定没有变化,进口井型电离室的最佳驻留位置在平坦峰值5mm范围内灵敏度变化为0.1%。结论该电离室优点:测量快速准确,同时可以测量192Ir,125I和103Pd等放射源活度。测量范围从3.7MBq～7.4×105MBq。该井型电离室将填补我国现场检测仪器的空白,并能形成我国有自主知识产权的产品。     

Consistency of vendor-specified activity values for 192Ir brachytherapy sources

A long-term comparison was done between the manufacturer-stated ¹⁹²Ir activity and the measured ¹⁹²Ir activities determined with a well-type ionization chamber. Sources for a Nucletron Micro Selectron high-dose-rate (HDR) unit were used for this purpose. The radioactive sources reference activities were determined using a PTW well-type ionization chamber traceable to the National Institute of Standards and Technology Primary Calibration Laboratory. The measurements were taken in a period of 56 months with 17 different radioactive sources. The manufacturer stated activities were taken from the source calibration certificate provided by the manufacturer. These values were compared with the measured activities. The results have shown that both the percentage deviation of the monthly control measurements with the well-type chamber and the ratio between the measured activities to the manufacturer-stated value lie within ± 2.5%. These results were compared with similar published data and with uncertainty level (3% of the mean and 5% maximum deviation from mean) for brachytherapy sources calibration recommended by the AAPM. It was concluded that a threshold level of ±2.5% can be used as a suitable quality assurance indicator to spot problems in our department. The typical ±5% uncertainty as provided by the manufacturers may be tightened to ±3% to be more in line with published AAPM reports.     

Calibration coefficient of reference brachytherapy ionization chamber using analytical and Monte Carlo methods

A cylindrical graphite ionization chamber of sensitive volume 1002.4 cm³ was designed and fabricated at Bhabha Atomic Research Centre (BARC) for use as a reference dosimeter to measure the strength of high dose rate (HDR) ¹⁹²Ir brachytherapy sources. The air kerma calibration coefficient (N_K) of this ionization chamber was estimated analytically using Burlin general cavity theory and by the Monte Carlo method. In the analytical method, calibration coefficients were calculated for each spectral line of an HDR ¹⁹²Ir source and the weighted mean was taken as N_K. In the Monte Carlo method, the geometry of the measurement setup and physics related input data of the HDR ¹⁹²Ir source and the surrounding material were simulated using the Monte Carlo N-particle code. The total photon energy fluence was used to arrive at the reference air kerma rate (RAKR) using mass energy absorption coefficients. The energy deposition rates were used to simulate the value of charge rate in the ionization chamber and N_K was determined. The Monte Carlo calculated N_K agreed within 1.77 % of that obtained using the analytical method. The experimentally determined RAKR of HDR ¹⁹²Ir sources, using this reference ionization chamber by applying the analytically estimated N_K, was found to be in agreement with the vendor quoted RAKR within 1.43%.     

Calibration of pre-cut iridium-192 wires for low dose rate interstitial brachytherapy using a Farmer-type ionization chamber

A technique, originally developed for calibrating small low activity caesium sources, which uses a Farmer-type ionization chamber, has been further developed for use with iridium wires. Correction factors have been generated to account for the finite source and detector sizes, and attenuation in the source carriers. The air kerma calibration factor for heavily filtered 280 kV X-rays was used for reference back to the National Standard. The results of this calibration method have been compared with the calibration figures given by the manufacturers over a 5 year period for the emissions from 50 batches of wires of varying strengths. Agreement to within +/- 3.2% was achieved in all cases, establishing that the method is satisfactory for acceptance testing purposes. The mean agreement was good to within 0.2%, but the possibility of a systematic error of between 1% and 3% existing both in this method and in the method used by the manufacturer is discussed.     

Comparison of air kerma rate between the S7500 and S7600 xoft axxent sources

PURPOSEThe purpose of this work was to evaluate differences in air-kerma rate of the older, S7500 water-cooled Xoft Axxent source and newer, S7600 Galden-cooled source.METHODS AND MATERIALSThe Attix Free Air Chamber (FAC) at the UWMRRC was used to measure the air-kerma rate at 50 cm for six S7600 Xoft Axxent sources. The average measured air-kerma of the S7600 sources was compared with the measured average air-kerma rate from five S7500 sources. The air-kerma rates of the S7500 sources were measured in a Standard Imaging HDR 1000+ well chamber. The FAC measurements were used to determine a well chamber calibration coefficient for the S7600 source. The S7500 calibration coefficients were incorrectly applied to the S7600 sources to indicate the magnitude of error that can occur if the incorrect calibration coefficient is used.RESULTSA 10.3% difference was observed between the average air-kerma rates of the two sources although a 17% difference was observed between their calibration coefficients. The application of the S7500 calibration coefficient to the S7600 sources resulted in measured air-kerma rates that were 20% greater than the true value.CONCLUSIONSThis work indicates the need for a new air-kerma rate standard for the S7600 sources, and the results presented in this work are indicative of values that would be obtained at National Institute of Standards and Technology.     

Comparison of high-dose-rate 192Ir source strength measurements using equipment with traceability to different standards

PurposeAccording to the American Association of Physicists in Medicine Task Group No. 43 (TG-43) formalism used for dose calculation in brachytherapy treatment planning systems, the absolute level of absorbed dose is determined through coupling with the measurable quantity air-kerma strength or the numerically equal reference air-kerma rate (RAKR). Traceability to established standards is important for accurate dosimetry in laying the ground for reliable comparisons of results and safety in adoption of new treatment protocols. The purpose of this work was to compare the source strength for a high-dose rate (HDR) ¹⁹²Ir source as measured using equipment traceable to different standard laboratories in Europe and the United States.Methods and MaterialsSource strength was determined for one HDR ¹⁹²Ir source using four independent systems, all with traceability to different primary or interim standards in the United States and Europe.ResultsThe measured HDR ¹⁹²Ir source strengths varied by 0.8% and differed on average from the vendor value by 0.3%. Measurements with the well chambers were 0.5% ± 0.1% higher than the vendor-provided source strength. Measurements with the Farmer chamber were 0.7% lower than the average well chamber results and 0.2% lower than the vendor-provided source strength. All of these results were less than the reported source calibration uncertainties (k = 2) of each measurement system.ConclusionsIn view of the uncertainties in ion chamber calibration factors, the maximum difference in source strength found in this study is small and confirms the consistency between calibration standards in use for HDR ¹⁹²Ir brachytherapy.     

Mechanical evaluation of the Bravos afterloader system for HDR brachytherapy

PurposeThe Bravos afterloader system was released by Varian Medical Systems in October of 2018 for high-dose-rate brachytherapy with ¹⁹²Ir sources, containing new features such as the CamScale (a new device for daily quality assurance and system recalibration), channel length verification, and different settings for rigid and flexible applicators. This study mechanically evaluated the Bravos system precision and accuracy for clinically relevant scenarios, using dummy sources.Methods and MaterialsThe system was evaluated after three sets of experiments: (1) The CamScale was used to verify inter- and intra-channel dwelling variability and system calibration; (2) A high-speed camera was used to verify the source simulation cable movement inside a transparent quality assurance device, where dwell positions, dwell times, transit times, speed profiles, and accelerations were measured; (3) The source movement inside clinical applicators was captured with an imaging panel while being exposed to an external kV source. Measured and planned dwell positions and times were compared.ResultsMaximum deviations between planned and measured dwell positions and times for the source cable were 0.4 mm for the CamScale measurements and 0.07 seconds for the high-speed camera measurements. Mean dwell position deviations inside clinical applicators were below 1.2 mm for all applicators except the ring that required an offset correction of 1 mm to achieve a mean deviation of 0.4 mm.ConclusionsFeatures of the Bravos afterloader system provide a robust and precise treatment delivery. All measurements were within manufacturer specifications.     

用治疗级电离室测量短脉冲高剂量率X射线的实验研究

目的 探索研究治疗级电离室用于短脉冲高剂量率X射线的快速测量。方法 利用内插法测量某电子加速器装置所致脉冲X射线半值层,估算其等效能量;采用治疗级电离室和热释光测量方法,对比设备周围同一方向不同距离处相同数量脉冲辐射的累积剂量;分析电离室剂量仪测量结果与源距离之间的关系,对比不同频率下同一位置相同数量脉冲辐射的累积剂量。结果 工作状态下,距设备外壁1~12 m累计接收100个脉冲辐射,热释光测得空气比释动能范围0.08~9.65 mGy,电离室剂量仪所测范围0.08~9.85 mGy,两者相差在10%以内;距设备正前方2 m处,不同频率（1~10 Hz）下,电离室剂量仪所测100个脉冲所致X射线空气比释动能无明显差异（P>0.05）。结论 在实验所涉加速器装置的剂量率和脉冲频率范围内,治疗级电离室剂量仪可用于短脉冲X射线辐射剂量的快速测量。     

Calibration of a Microselectron HDR iridium 192 source

A method for the calibration of the output, in terms of an air kerma rate, of the high activity miniature iridium 192 sources used in the Microselectron HDR afterloading unit is described. An air kerma rate is measured using a calibrated thimble chamber in an "in-air" calibration jig. The results are compared with an air kerma rate derived from the manufacturer's test certificate. In some cases, the ionization chamber measurements have been followed by a further calibration check using thermoluminescent dosimetry. Other checks carried out when a new source is received are also briefly described.     

Study of scattered radiation for in-air calibration by a multiple-distance method using ionization chambers and an HDR 192Ir brachytherapy source

The aim of this study is to estimate the room-scatter correction when measuring air kerma rate of an HDR 192Ir brachytherapy source by in-air calibration. The variation in scattered radiation due to the specially designed jig and from the room walls was also studied. Two therapy ion chambers of volume 0.1 cm3 and 0.6 cm3 were used in the present study. Air kerma was measured by placing the source at several distances between 10 cm and 20 cm from the chamber. The scatter radiation was determined by superimposing the theoretically derived model curve of known scatter (based on the inverse square law) over the plot of measured air kerma strength values. The scatter radiation was estimated in terms of percentage of the primary radiation at 10 cm measurement distance. The scatter estimated by the 0.6 cm3 chamber at two positions was 0.33% and 0.59%, respectively. Similarly the scatter estimated at two other positions by the 0.1 cm3 chamber was 0.58% and 1.11%. This variation in scatter with position as well as with the chamber was due to the varying scatter contribution from components of the measurement set-up. The scatter radiation becomes constant at a distance greater than 100 cm from the walls of the room. We conclude that a fixed chamber with changing source positions should be used in multiple-distance measurement of air kerma rate when using a measurement jig.     

Air kerma and absorbed dose standards for reference dosimetry in brachytherapy

This article reviews recent developments in primary standards for the calibration of brachytherapy sources, with an emphasis on the currently most common photon-emitting radionuclides. The introduction discusses the need for reference dosimetry in brachytherapy in general. The following section focuses on the three main quantities, i.e. reference air kerma rate, air kerma strength and absorbed dose rate to water, which are currently used for the specification of brachytherapy photon sources and which can be realized with primary standards from first principles. An overview of different air kerma and absorbed dose standards, which have been independently developed by various national metrology institutes over the past two decades, is given in the next two sections. Other dosimetry techniques for brachytherapy will also be discussed. The review closes with an outlook on a possible transition from air kerma to absorbed dose to water-based calibrations for brachytherapy sources in the future.Successful radiotherapy requires an accurate measurement of the radiation source output as part of a crucial quality assurance (QA) programme, as recommended by both the American Association of Physicists in Medicine (AAPM) Task Group No. 56 (TG-56)¹ and the European Society for Radiotherapy and Oncology (ESTRO).² Radioactive brachytherapy sources used for cancer treatment need to be calibrated at radiotherapy centres before clinical use. The purpose of the independent source calibration at the clinic is to verify the source strength stated on the vendor''s source calibration certificate,³ to ensure traceability to appropriate national or international standards and to make sure that dose measurements between different radiotherapy centres are consistent.² This enables clinicians to compare treatment techniques for specified radiation doses with the aim to improve the treatment outcome for cancer patients. The AAPM TG-56 report¹ recommends brachytherapy dose delivery accuracy within 5–10% with source calibration accuracy within 3%. The expanded uncertainties quoted here are based on a standard uncertainty multiplied by a coverage factor k = 2 (two standard deviations), providing a coverage probability of approximately 95%. Many components contribute to the overall uncertainty in the radiation dose delivered by brachytherapy sources and the target uncertainty of <10% (k = 2) may be difficult to achieve.⁴ The aim, however, is to keep the uncertainty in the delivered dose at the lowest possible level, which requires all dosimetric practices to be optimized. Accurate knowledge of the source strength is one of the steps in the dosimetry chain.⁵The source strength of brachytherapy sources can be measured with traceably calibrated radiation dosemeters. The calibration chain starts at the national standards laboratories, which develop and maintain primary standards for radiation qualities used in clinics. The primary standards are instruments of the highest metrological quality, which realize physical quantities from first principles.⁶ The accuracy of the primary standards is verified by comparison with similar standards of other laboratories, which are part of the international measurement system. Primary standard instruments can be very complex and too awkward to be used for routine measurements in the hospital environment. Commercially available secondary standard dosemeters, such as well-type ionization chambers,⁷ are more suitable and practical for QA measurements of brachytherapy sources in the clinic. These instruments can be traceably calibrated against a primary standard [either directly at a primary standards dosimetry laboratory (PSDL) or via a secondary standards dosimetry laboratory] and subsequently used by hospital physicists or source vendors to measure the source strength of brachytherapy sources.The brachytherapy source strength is an important input parameter for the treatment planning system (TPS), which calculates the dose distribution in tissue close to the radiation source.This review article is concerned with reference dosimetry for sealed brachytherapy photon sources that involves measuring either the air kerma rate at a reference distance of 1 m or the absorbed dose rate to water at a reference distance of 1 cm from the centre of the source. The article focuses on recent trends in the development of primary standards for three of the currently most commonly used photon-emitting brachytherapy source types, low-dose-rate (LDR) ¹²⁵I and ¹⁰³Pd seeds and high-dose-rate (HDR) ¹⁹²Ir sources. The discussion of standards for β-emitting brachytherapy sources is outside the scope of this review. Further details on primary standards for other photon-emitting and β-emitting brachytherapy sources can be found in a comprehensive review paper by Soares et al.⁸     

Intercomparison of ionisation chamber measurements from (125)I seeds.

The reference air kerma rates of a set of individual (125)I seeds were calculated from current measurements of a calibrated re-entrant ionisation chamber. Single seeds were distributed to seven Australian brachytherapy centres for the same measurement with the user's instrumentation. Results are expressed as the ratio of the reference air kerma rate measured by the Australian Nuclear Science & Technology Organisation (ANSTO) to the reference air kerma rate measured at the centre. The intercomparison ratios of all participants were within +/-5% of unity.     

测量后装近距离放射治疗192Ir源参考空气比释动能方法学研究

目的 用NE2570剂量仪,2571指形电离室,测量 ¹⁹²Ir源空气比释动能支架,测量1m处 ¹⁹²Ir源参考空气比释动能。 方法将测量支架放在离墙、地面1m处,指形电离室插入有机玻璃测量支架夹具中,源中心距电离室中心的最佳距离是16cm, 源通过后装机的传输系统传输到施源器中,测量源参考空气比释动能。根据 ⁶⁰Co γ射线,250 kVΧ射线空气照射量刻度因子换算为空气比释动能刻度因子,再由内插公式计算 ¹⁹²Ir源空气比释动能刻度因子。对墙、地、空气、测量支架的散射校准因子,通过阴影屏蔽实验得到;对初始光子减弱校准因子;电离室壁产生的电子非均匀校准因子,均由IAEA的1079号报告(近距离放射治疗源的刻度)中查表得到。 结果在相同环境条件下,使用2种测量方法,指形电离室测量 ¹⁹²Ir源空气比释动能,经转换系数计算源外观活度为1.584×10¹¹Bq;井型电离室测量 ¹⁹²Ir源空气比释动能强度,经转换系数计算源外观活度为1.561×10¹¹ Bq,2个结果的相对偏差为 1.4%。结论指形电离室测量源空气比释动能,该物理量与源的结构、尺寸、壳材料、电离室形状、材质和尺寸无关,测量源空气比释动能与源的空气照射量比较,不确定度误差小。     

蒙特卡罗方法计算后装192Ir源散射校正因子研究

目的 研究一种方便、可行地推算医用后装192Ir源空气比释动能散射校正因子的方法,便于医院用指形电离室进行活度测量的QA工作的开展.方法 用指形电离室测量有铅挡块和无铅挡块192Ir源空气比释动能,根据国际原子能机构(IAEA)1079号报告,计算192Ir源散射校正因子.用蒙特卡罗(MC)方法模拟测量条件,计算192Ir源散射校正因子,并与实验结果进行比较验证,同时模拟几种不同电离室型号和房间尺寸,计算并给出不同192Ir源散射校正因子.结果 蒙特卡罗方法模拟192Ir源散射校正因子与实验测得的散射校正因子比较,相对误差为0.8%.利用蒙卡计算得散射校正因子推算出的源活度和用井型电离室测量推算出的源活度相差2.4%.MC模拟IAEA1079号报告中的两种球形电离室计算结果与报告中给出的结果比较,相对误差范围在0.3%～0.4%.模拟5种不同型号指形电离室,不同房间尺寸,相对误差范围在3%之内.结论 用蒙卡方法模拟计算后装192Ir源散射校正因子的方法是可行的,此方法方便了医院用指形电离室进行近距离治疗QA工作的开展.

Abstract：

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.     

Flexitron后装机的临床调试

目的 为满足国家标准和临床需求,对Flexitron后装机的硬件和软件制定临床调试流程、项目内容和测试方法,并建立相关的质量控制规程。方法 临床调试分为硬件、治疗计划系统(TPS)和端对端(ETE)的全流程测试。采用源位置检测标尺测量放射源的到位精度;通过秒表计时、电离室测量、视频分析3种方法检测驻留时间的精度和线性;使用高精度尺测试模拟尺、连接管、源位置检测标尺等测量工具的精度;使用胶片校准标记丝和施源器;使用静电计和井型电离室校准放射源活度。通过实物图像对TPS的显示、重建精度进行评价。采用自制模体完成扫描、计划和剂量测量进行ETE测试。结果 调试项目中的精度检测结果均在可接受的限值之内,源活度测量结果偏差为0.21%,ETE点剂量测量偏差为2.32%,均满足临床使用要求。但精度检测项目中,核磁标记丝的标称和实测值存在2 mm差异,因此基于核磁影像采用标记丝进行管道重建时需要修正。结论 本研究通过总结Flexitron后装机的临床调试经验,制定了后装机、计划系统各项目的质量控制方法及结果的基线水平,为后装机投入临床使用前的调试工作提供了参考。