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.     

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.     

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。     

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.     

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。     

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%.     

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。     

Evaluation of Scatter Contribution and Distance Error by Iterative Methods for Strength Determination of HDR 192Ir Brachytherapy Source

High-dose rate (HDR) ¹⁹²Ir brachytherapy sources are commonly used for management of malignancies by brachytherapy applications. Measurement of source strength at the hospital is an important dosimetry requirement. The use of 0.6-cm³ cylindrical ionization chamber is one of the methods of measuring the source strength at the hospitals because this chamber is readily available for beam calibration and dosimetry. While using the cylindrical chamber for this purpose, it is also required to determine the positioning error of the ionization chamber, with respect to the source, commonly called a distance error (c). The contribution of scatter radiation (M_s) from floor, walls, ceiling, and other materials available in the treatment room also need to be determined accurately so that appropriate correction can be applied while calculating the source strength from the meter reading. Iterative methods of Newton-Raphson and least-squares were used in this work to determine scatter contribution in the experimentally observed meter reading (pC/s) of a cylindrical ionization chamber. Monte Carlo simulation was also used to cross verify the results of the least-squares method. The experimentally observed, least-squares calculated and Monte Carlo estimated values of meter readings from HDR ¹⁹²Ir brachytherapy source were in good agreement. Considering procedural simplicity, the method of least-squares is recommended for use at the hospitals to estimate values of f (constant of proportionality), c, and M_s required to determine the strength of HDR ¹⁹²Ir brachytherapy sources.     

A determination of the half-life of 137Cs

The half-life of ¹³⁷Cs has been found to be 10,940.8±6.9 d where the uncertainty combines components of Type A and B equivalent to one standard deviation. A ¹³⁷Cs source was measured in an ionization chamber for 3000 d relative to a well define ²²⁶Ra reference source.     

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.     

Direct measurement of 60Co and 125I activity by the sum-peak method in PTKMR-BATAN

The sum-peak method was successfully applied to the determination of the activity of ⁶⁰Co and ¹²⁵I sources measured by HPGe and well-type NaI(Tl) detectors in PTKMR-BATAN. The result of the ⁶⁰Co activity agrees with the activity value measured by using a calibrated ionization chamber within a range of about 0.35–0.5% and the activity result of ¹²⁵I agrees with the activity value measured using the photon–photon coincidence method within a range of 0.05–0.26%.     

Control of refractory lining wear by using radioisotopes

The use of radioactive isotopes as tracers provides a means of measuring the refractory wear rate in a converter or blast furnace.The paper reports on the research carried out for the implementation of this technique in the Portugues Steelmaking Company (Siderurgia Nacional, EP).Sources of 7.4 × 10⁷ Bq ¹⁹²Ir and ⁶⁰Co have been installed in holes made in the refractory bricks at suitable positions. The specific activity of the steel with dissolved sources is lower than 1.5 × 10³ Bq kg⁻¹.Both radioisotopes prove to be suitable, but ¹⁹²Ir is a better choice. The results also show that the wear rate during a campaign depends on the working conditions and is not constant. Values between 0.49 and 0.74 mm/charge have been measured.     

免冲洗胶片法测量后装机192Ir放射源到位精度及步进距离精度

目的 研究使用免冲洗胶片测量后装机¹⁹²Ir放射源到位精度及步进距离精度的方法。方法 使用GAFCHROMIC^® EBT³型免冲洗胶片对1台国产后装机¹⁹²Ir放射源到位精度及步进距离精度进行测量。曝光后的胶片使用EPSON PREFACTION V700 PHOTO胶片扫描仪扫描成胶片分析软件要求的图像格式,然后用SNC Patient 5.2软件中的胶片分析软件对图像进行分析。 结果 以源活性中心为基准点,该后装机使用胶片法测量的¹⁹²Ir放射源到位精度为-0.75 mm。免冲洗胶片分析方法能够分辨2个驻留点间5 mm的步进距离精度,不能够分辨2个驻留点间2.5 mm的步进距离精度;2.5 mm步进距离精度可通过测量3个连续驻留点第1点和第3点间距离是否为5 mm的方法进行间接判断。该后装机使用胶片法测量的连续9个驻留点间的5 mm步进距离无偏差;2.5 mm步进距离精度间接判断结果无偏差。使用胶片法测量的后装机¹⁹²Ir放射源到位精度符合国家标准要求。结论 免冲洗胶片方法可以用于后装机¹⁹²Ir放射源到位精度及步进距离精度的测量。     

国产血管内~(192)Ir线源的放射剂量测定

目的　对国产血管内192 Ir线源的剂量分布进行评价 ,为动物实验和临床应用提供依据。方法　采用KodakX omatV慢感光胶片 ,从平行和垂直于放射源长轴方向进行测量 ,径向测量时间为 2 5、45、6 5和 82s ,轴向测定时间为 2 5s,同时进行标准剂量的标定 ,通过胶片自动分析测量系统分析剂量分布和吸收剂量。参考AAPMTGNo.6 0报告 ,采用MonteCarlo方法对放射源的辐射剂量进行理论计算 ,同时与采用AAPMTGNo.43报告计算方法进行比较。结果　国产血管内192 Ir线源具有良好的剂量分布。AAPMTGNo .43报告计算方法比MonteCarlo方法高估 32 %的辐射剂量。结论国产192 Ir线源作为血管内放射源是可行的 ,采用慢感光胶片测定放射源的剂量分布是一种有效手段。     

HDR brachytherapy well chamber calibration and stability evaluated over twenty years of clinical use

PurposeThe purpose of the study was to establish, using a retrospective analysis of existing hospital records, the long-term stability and accuracy of a high-dose-rate brachytherapy well chamber. This should be assessed to determine reliability and appropriate calibration frequency. The accrual of long-term data that demonstrates the stability of our chamber may inform others of the performance they might expect from similar equipment.Methods and MaterialsWe evaluated air kerma strength measurements made with the PTW 32002 (Nucletron 077.091) high-dose-rate well chamber on 72 ¹⁹²Ir sources over an 18-year period and the seven calibrations of that chamber which span a 27-year period.ResultsConsecutive air kerma strength measurements agreed within 0.01% on average. The chamber measurement agreed with the source specification within 0.02% on average, but was up to 1.4% during some calibration periods. The chamber calibration coefficient varied by a maximum of 5% over seven chamber calibration measurements.ConclusionsThe constancy of the well chamber current compared with the source manufacturer suggests that our chamber has been stable to better than 2% over a period of 18 years. Although the chamber has received different calibration coefficients over time, these coefficients are within the combined uncertainties of any two calibrations and are consistent with the chamber being stable. The agreement we have observed between clinical measurements and the source manufacturer would justify an action level for further investigation of 1%, for this specific chamber.     

Monte Carlo calculations of calibration and geometry correction factors of a radionuclide calibrator.

The activity of radioactive pharmaceuticals administered to patients in nuclear medicine is usually determined using well-type high-pressure ionization chambers. For the Bqmeter chamber (Consortium BQM, Czech Republic) a Monte Carlo model was created using the MCNP4C2 code. Basic chamber characteristics for two sample containers of various geometry (a vial and an ampoule) were calculated and compared with measurements. As the pharmaceuticals are often measured in various syringes, the chamber response for samples in syringes was also studied.     

Assessment of dose uniformity around high dose rate <Superscript>192</Superscript>Ir and <Superscript>60</Superscript>Co stepping sources

This study aimed to evaluate dose uniformity for ¹⁹²Ir and ⁶⁰Co stepping sources. High dose rate ¹⁹²Ir and ⁶⁰Co stepping sources were simulated by the MCNPX Monte Carlo code. To investigate dose uniformity, treatment lengths of 30, 50, 100, and 150 mm with stepping distances of 3, 5, 7, and 10 mm were considered. Finally, dose uniformity for the ¹⁹²Ir and ⁶⁰Co stepping sources with increasing distances from the source were assessed at these treatment lengths and steps. The findings showed that the dose distribution was non-uniform for regions in close vicinity of the source, especially in the high source steps, but for most points at distances >10 mm from the center of the source, the dose distribution was uniform. For most points, the dose uniformity increased with reduction of the source steps and increments of the transverse distance from the source. The dose non-uniformity was similar for most of the corresponding points of ⁶⁰Co and ¹⁹²Ir sources with the same treatment lengths and source steps, except at the distance of 150 mm. When using stepping technique for the treatment of tumors, more attention should be focused on treatment planning, especially with higher stepping distances and lower transverse distances from the source.     

Intracavitary dosimetry of a high-activity remote loading device with oscillating source

Dosimetric experiments have been carried out in order to obtain the dose distribution in water around a Fletcher applicator loaded by a Buchler system containing two 137Cs 148 GBq (4 Ci) sources and one 192Ir 740 GBq (20 Ci) source. The mechanical system which controls the movement of the 192Ir source and the resulting motion of the source are described. The dose distribution around the sources was measured photographically and by a PWT Normal 0.22 cm3 ionisation chamber. The absolute dose rate was measured along the lateral axes of the sources. The measurements of exposure in water near the sources were corrected for the effect due to the finite volume of the chamber. The "quantisation method" described by Cassell (1983) was utilised to calculate the variation of the dose rate along the lateral axes of the sources. The dose distribution around both 192Ir and 137Cs sources was found to be spherical for angles greater than 40 degrees from the longitudinal axes of the sources. A simple algorithm fitting the data for the moving 192Ir source is proposed. A program written in FORTRAN IV and run on a Univac 1100/80 computer has been used to plot dose distributions on anatomical data obtained from CT images. The uncertainties of the measurements and calculations have been examined and the greatest error has been found to be 5.5%. The clinical significance of the treatment method is discussed.     

A Monte Carlo brachytherapy study for dose distribution prediction in an inhomogeneous medium

The purpose of this study was to present a theoretical analysis of how the presence of bone in interstitial brachytherapy affects dose rate distributions. This study was carried out using a Monte Carlo simulation of the dose distribution in homogeneous medium for 3 commonly used brachytherapy seeds. The 3 seeds investigated in this study are iridium-192 (¹⁹²Ir) iodine-125 (¹²⁵I), and palladium-103 (¹⁰³Pd). The computer code was validated by comparing the specific dose rate (Λ), the radial dose function g(r), and anisotropy function F(r,θ) for all 3 seeds with the AAPM TG-43 dosimetry formalism and current literature. The ¹⁹²Ir seed resulted in a dose rate of 1.115 ± 0.001 cGy-hr⁻¹-U⁻¹, the ¹²⁵I seed resulted in a dose rate of 0.965 ± 0.006 cGy/h⁻¹/U⁻¹, and the ¹⁰³Pd seed resulted in a dose rate of 0.671 ± 0.002 cGy/h⁻¹/U⁻¹. The results for all 3 seeds are in good agreement with the AAPM TG-43 and current literature. The validated computer code was then applied to a simple inhomogeneous model to determine the effect bone has on dose distribution from an interstitial implant. The inhomogeneous model showed a decrease in dose rate of 2% for the ¹⁹²Ir, an increase in dose rate of 84% for ¹²⁵I, and an increase in dose rate of 83% for the ¹⁰³Pd at the surface of the bone nearest to the source.     

Dosimetry of a sonolucent material for an ultrasound-compatible gynecologic high-dose-rate brachytherapy cylinder using Monte Carlo simulation and radiochromic film

Purposehe purpose of this study was to study the dosimetric characterization of sonolucent material “TPX” to be used toward gynecologic high-dose-rate brachytherapy treatments using ultrasound-compatible cylinders in non–model-based dose calculation workflows.MethodsMonte Carlo simulations were performed using EGSnrc application egs_brachy in cylinders of polymethylpentene (TPX) plastic, water, and PMMA. Simulations were performed of five 192Ir sources placed longitudinally in ～3.7 cm diameter, 5.0 cm length cylinders (matching physical cylinders used in film measurements). TPX and PMMA dose distributions and percentage depth dose curves were compared relative to water.Film measurements were performed to validate egs_brachy simulations. TPX and PMMA cylinders were placed in a water tank using 3D-printed supports to position film radially and touching the surface of the cylinders. The same five 192Ir dwell positions were delivered as simulated in egs_brachy.ResultsThe egs_brachy and film percentage depth doses agreed within film uncertainties. The egs_brachy relative dose difference between TPX and water was (0.74 ± 0.09)% and between PMMA and water was (-0.79 ± 0.09)% over the dose scoring phantom. Dose differences for TPX and PMMA relative to water were less than ± 1% within 5 cm of the cylinder surface.ConclusionsIn a solid sonolucent sheath of TPX, the dosimetric differences are comparable with PMMA and other applicator materials in clinical use. No additional uncertainty to dose calculation is introduced when treating through TPX cylinders compared with current applicator materials, and therefore, it is acceptable to perform gynecologic brachytherapy treatments with a sonolucent sheath inserted during radiation delivery.