Activity measurements of 18F and 90Y with commercial radionuclide calibrators for nuclear medicine in Switzerland

The activity of radiopharmaceuticals in nuclear medicine is measured before patient injection with radionuclide calibrators. In Switzerland, the general requirements for quality controls are defined in a federal ordinance and a directive of the Federal Office of Metrology (METAS) which require each instrument to be verified. A set of three gamma sources (Co-57, Cs-137 and Co-60) is used to verify the response of radionuclide calibrators in the gamma energy range of their use. A beta source, a mixture of ⁹⁰Sr and ⁹⁰Y in secular equilibrium, is used as well. Manufacturers are responsible for the calibration factors. The main goal of the study was to monitor the validity of the calibration factors by using two sources: a ⁹⁰Sr/⁹⁰Y source and a ¹⁸F source. The three types of commercial radionuclide calibrators tested do not have a calibration factor for the mixture but only for ⁹⁰Y. Activity measurements of a ⁹⁰Sr/⁹⁰Y source with the ⁹⁰Y calibration factor are performed in order to correct for the extra-contribution of ⁹⁰Sr. The value of the correction factor was found to be 1.113 whereas Monte Carlo simulations of the radionuclide calibrators estimate the correction factor to be 1.117. Measurements with ¹⁸F sources in a specific geometry are also performed. Since this radionuclide is widely used in Swiss hospitals equipped with PET and PET-CT, the metrology of the ¹⁸F is very important. The ¹⁸F response normalized to the ¹³⁷Cs response shows that the difference with a reference value does not exceed 3% for the three types of radionuclide calibrators.     

Standardization of 18F and its use for the Romanian PET metrological traceability chain assurance

This paper presents the work performed at IFIN-HH and at the users’ sites, aimed to establish the ¹⁸FDG Romanian PET traceability chain. It summarizes the operations: (i) Absolute standardization of a ¹⁸F solution, by the 4πβ(PC)-γ coincidence method; (ii) Control of gamma-ray impurity content and measurement of the activity by gamma ray spectrometry; (iii) Measurement of the half life, for detection of some short/long life positron emitter impurities; (iv) Calibration of the CENTRONIC IG12/20A ionization chamber; (v) Calibration of commercial radionuclide calibrators.     

A quality control exercise of radionuclide calibrators among Belgian hospitals

On the initiative of the Belgian Association of Hospital Physicists, eleven Belgian hospitals participated in a quality control of radionuclide calibrators conducted in collaboration with the Central Bureau for Nuclear Measurements of the Commission of the European Communities. For practical reasons the nuclide⁵⁷Co was chosen. The results from 20 different radionuclide calibrators show a fair agreement with a similar comparison carried out in 1980 in the UK.     

Comparison of 99Tcm and 131I in Polish hospitals, 2007

The Radioisotope Centre (RC) has organized for the first time in Polish hospitals a comparison of activity measurements of ⁹⁹Tc^m and ¹³¹I. From the 58 Polish hospitals invited, 37 participated in the comparison. Each participant got about 4 mL of ⁹⁹Tc^m eluate and two ¹³¹I capsules, low- and high-activity. Sources were standardized in the RC before dispatch using a 4π ionization chamber calibrated by standard solutions. Participants used their own radionuclide calibrators. The measurement results were evaluated using the E_n criterion and the z′-score. From all 118 measurement results received from participants, 75 results (64%) lay within ±5% of the RC value, and 99 results (84%) lay within ±10.0%.     

Direct simultaneous production of [15O]water and [13N]ammonia or [18F]fluoride ion by 26 MeV proton irradiation of a double chamber water target

A simple double liquid chamber target was developed to provide the option for simultaneous production of [¹⁵O]H₂O and either ¹³N or ¹⁸F using a single proton beam. Irradiation of natural water in a thin aluminium front chamber produced [¹⁵O]H₂O by the ¹⁶O(p, pn)¹⁵O reaction directly. Large (0.5–1.0 Ci) doses of sterile [¹⁵O]H₂O (> 99.95% radionuclide purity) were routinely prepared in 1 min from end of 20 μA bombardments using this target and an in-line mixed bed ion exchange column purification. Water in the thin front chamber degraded proton energies on exit to 20-18 MeV. The rear silver liquid chamber was threefold thick to 17 MeV protons in water and it efficiently produced either ¹³N by the ¹⁶O(p, α)¹³N reaction or [¹⁸F]fluoride ion by the ¹⁸O(p, n)¹⁸F reaction. Both target chambers were overpressurized with at least 6 atm of gas to minimize boiling/cavitation of water at high beam currents. Using hydrogen as the overpressure gas on the back chamber and an in-line anion exchange column radionuclidic cleanup process, high yields of sterile, aqueous [¹³N]NH₃ (40–200 mCi; 20 μA) were produced directly from the back chamber at the same time that [¹⁵O]H₂O was being produced from the front chamber. The combination of this target system with a cyclotron capable of generating 26–30 MeV protons provides great flexibility and simplicity for rapid, high volume production of the three best validated and most widely used radiopharmaceuticals at the present time in clinical positron emission tomography: [¹⁵O]H₂O, [¹³N]NH₃ and [¹⁸F]FDG.     

Comparison of the activity measurements in nuclear medicine services in the Brazilian northeast region

The Northeastern Regional Centre for Nuclear Sciences (CRCN-NE), National Nuclear Energy Commission, has organized for the first time in nuclear medicine services (NMSs) in the Brazilian northeast region a comparison of activity measurements for ^99mTc, ¹³¹I, ⁶⁷Ga, ²⁰¹Tl and ⁵⁷Co. This tool is widely utilized to evaluate not only the accuracy of radionuclide calibrators, but also the competence of NMSs to measure the activity of the radiopharmaceuticals and the performance of the personnel involved in these measurements. The comparison results showed that 90% of the results received from participants are within the ±10% limit established by the Brazilian Norm.     

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%.     

On line high dose static position monitoring by ionization chamber detector for industrial gamma irradiators

A 1 cm³ cylindrical ionization chamber was developed to measure high doses on line during the sample irradiation in static position, in a ⁶⁰Co industrial plant. The developed ionization chamber showed to be suitable for use as a dosimeter on line. A good linearity of the detector was found between the dose and the accumulated charge, independently of the different dose rates caused by absorbing materials.     

A patient-specific quality assurance study on absolute dose verification using ionization chambers of different volumes in RapidArc treatments

The recalculation of 1 fraction from a patient treatment plan on a phantom and subsequent measurements have become the norms for measurement-based verification, which combines the quality assurance recommendations that deal with the treatment planning system and the beam delivery system. This type of evaluation has prompted attention to measurement equipment and techniques. Ionization chambers are considered the gold standard because of their precision, availability, and relative ease of use. This study evaluates and compares 5 different ionization chambers: phantom combinations for verification in routine patient-specific quality assurance of RapidArc treatments. Fifteen different RapidArc plans conforming to the clinical standards were selected for the study. Verification plans were then created for each treatment plan with different chamber-phantom combinations scanned by computed tomography. This includes Medtec intensity modulated radiation therapy (IMRT) phantom with micro-ionization chamber (0.007 cm³) and pinpoint chamber (0.015 cm³), PTW-Octavius phantom with semiflex chamber (0.125 cm³) and 2D array (0.125 cm³), and indigenously made Circular wax phantom with 0.6 cm³ chamber. The measured isocenter absolute dose was compared with the treatment planning system (TPS) plan. The micro-ionization chamber shows more deviations when compared with semiflex and 0.6 cm³ with a maximum variation of ?4.76%, ?1.49%, and 2.23% for micro-ionization, semiflex, and farmer chambers, respectively. The positive variations indicate that the chamber with larger volume overestimates. Farmer chamber shows higher deviation when compared with 0.125 cm³. In general the deviation was found to be <1% with the semiflex and farmer chambers. A maximum variation of 2% was observed for the 0.007 cm³ ionization chamber, except in a few cases. Pinpoint chamber underestimates the calculated isocenter dose by a maximum of 4.8%. Absolute dose measurements using the semiflex ionization chamber with intermediate volume (0.125 cm³) shows good agreement with the TPS calculated among the detectors used in this study. Positioning is very important when using smaller volume chambers because they are more sensitive to geometrical errors within the treatment fields. It is also suggested to average the dose over the sensitive volume for larger-volume chambers. The ionization chamber-phantom combinations used in this study can be used interchangeably for routine RapidArc patient-specific quality assurance with a satisfactory accuracy for clinical practice.     

Comparative measurements in dose calibrators and a gamma-ray spectrometer

Specific activity results of ⁵¹Cr, ⁶⁷Ga, ^99mTc and ²⁰¹Tl radioactive solutions, measured in different dose calibrators located at nuclear medicine services and in a calibrated HPGe gamma spectrometer, were compared. The HPGe spectrometer was calibrated in a well defined geometry by means of ⁶⁰Co, ¹³³Ba, ¹⁵²Eu, ^166mHo and ²⁴¹Am sources, previously standardized in a 4πβ?γ coincidence system. Despite the observed differences, the results may be accepted within the 10% uncertainty range, established by Brazilian regulatory standards.     

Preparation of pure tritium for a liquid D2/T2 target of muon-catalyzed fusion experiments

To prepare a pure liquid D₂/T₂ target for muon-catalyzed fusion (μCF) experiments, 50 TBq of tritium gas was purified with gas chromatography for isotopic enrichment and with the tritide formation reaction of uranium for chemical purification. The isotopic purity of the processed tritium was as high as 99.9% and the chemical purity 99.7%. The ³He content was 0.02% or less immediately after the preparation.A 300 mL (30 TBq) portion of pure T₂ gas was mixed with 700 mL of pure D₂ gas to be loaded into a target chamber made of stainless steel. The D₂/T₂ mixed gas was liquefied in the target chamber by cooling with liquid helium and irradiated with pulsed μ⁻ beams for more than 200 h. No characteristic muonic x-rays arising from impurities in the D₂/T₂ target were observed.     

Dose calibrator manufacturer-dependent bias in assays of 123I

Calibration factors for commercial ionization chambers (i.e. dose calibrators) were determined for a solution of ¹²³I; the activity was based on the 1976 NBS standard. A link between the NIST standard and the International Reference System (SIR) was established. The two major U.S. dose calibrator manufacturers recommend oppositely biased calibration factors, giving a spread of 11.3% in measured activities. With modern quantitative imaging techniques capable of ≤10% accuracy, this bias for a SPECT nuclide is highly significant.     

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。     

Modern dosimetric tools for 60Co irradiation at high containment laboratories

Abstract

Purpose: To evaluate an innovative photo-fluorescent film as a routine dosimetric tool during ⁶⁰Co irradiations at a high containment biological research laboratory, and to investigate whether manufacturer-provided chamber exposure rates can be used to accurately administer a prescribed dose to biological specimens.

Materials and methods: Photo-fluorescent, lithium fluoride film dosimeters and National Institutes of Standards and Technology (NIST) transfer dosimeters were co-located in a self-shielded ⁶⁰Co irradiator and exposed to γ-radiation with doses ranging from 5–85 kGy. Film dose-response relationships were developed for varying temperatures simulating conditions present when irradiating infectious biological specimens. Dose measurement results from NIST transfer dosimeters were compared to doses predicted using manufacturer-provided irradiator chamber exposure rates.

Results: The film dosimeter exhibited a photo-fluorescent response signal that was consistent and nearly linear in relationship to γ-radiation exposure over a wide dose range. The dosimeter response also showed negligible effects from dose fractionization and humidity. Significant disparities existed between manufacturer-provided chamber exposure rates and actual doses administered.

Conclusion: This study demonstrates the merit of utilizing dosimetric tools to validate the process of exposing dangerous and exotic biological agents to γ-radiation at high containment laboratories. The film dosimeter used in this study can be utilized to eliminate potential for improperly administering γ-radiation doses.     

Verification of the dose attenuation of a newly developed vacuum cushion for intensity-modulated radiation therapy of prostate cancer

This study measured the dose attenuation of a newly developed vacuum cushion for intensity-modulated radiation therapy (IMRT) of prostate cancer, and verified the effect of dose-correction accuracy in a radiation treatment planning system (RTPS). The new cushion was filled with polystyrene foams inflated 15-fold (Sφ ≒ 1 mm) to reduce contraction caused by air suction and was compared to normal polystyrene foam inflated to 50-fold (Sφ ≒ 2 mm). The dose attenuation at several thicknesses of compression bag filled with normal and low-inflation materials was measured using an ionization chamber; and then the calculated RTPS dose was compared to ionization chamber measurements, while the new cushion was virtually included as region of interest in the calculation area. The dose attenuation rate of the normal cushion was 0.010 %/mm (R ² = 0.9958), compared to 0.031 %/mm (R ² = 0.9960) in the new cushion. Although the dose attenuation rate of the new cushion was three times that of the normal cushion, the high agreement between calculated dose by RTPS and ionization chamber measurements was within approximately 0.005 %/mm. Thus, the results of the current study indicate that the new cushion may be effective in clinical use for dose calculation accuracy in RTPS.     

Comparison of calibration results for an extrapolation chamber obtained with different Sr+Y secondary standard sources

Two secondary standard systems of beta radiation were used to calibrate a PTW extrapolation chamber Model 23391. Three ⁹⁰Sr+⁹⁰Y sources of different activities were used in this calibration procedure. Medium-term stability of the response of the chamber was also studied. The calibration was performed with and without field-flattening filters. The relative standard deviation of the obtained calibration factors was 8.3% for the aluminum collecting electrode and 4.1% for the graphite collecting electrode.     

Precise practical measurements of 90Y used in radiopharmaceuticals using re-entrant ionization chambers

For diagnostic nuclear medicine, radiopharmaceuticals are labeled with γ-ray emitting nuclides. Routine activity measurements for these drugs are mainly performed with re-entrant ionization chambers. For radiotherapeutic applications, β-ray emitting nuclides are also used. Since very few gamma transitions are involved in these nuclides, an ionization chamber responds only to weak bremsstrahlung. In order to apply the re-entrant chamber method to routine activity assay for these β-ray emitting nuclides, detailed studies were carried out. It was determined that the response was strongly affected by the choice of thickness of the inner wall of the well in the measurement of high energy beta particle emitting nuclides. The geometrical dependences were successfully reduced. Accordingly, routine assay of radioactivity of (90)Y therapeutic pharmaceuticals can be performed within an acceptable uncertainty with ordinary re-entrant dose calibrators that are commercially available and equipped in most hospitals.     

四种剂量探测器对放疗加速器常用剂量率的响应特性分析

目的 探讨4种剂量探测器对放疗加速器常用剂量率的响应特性。方法 在100~600 cGy/min的剂量率范围内,分别测量PTW的0.6 cm³电离室、0.015 cm³电离室以及Matrixx Evolution 二维电离室矩阵和MapCHECK 二维半导体矩阵在同一测量条件和剂量下的剂量率响应;测量和分析0.6 cm³电离室在不同能量和工作电压下的剂量率响应特性。结果 PTW的0.6 cm³电离室、0.015 cm³电离室以及Matrixx Evolution 二维电离室矩阵在6 MV X射线能量下都表现一定的剂量率依赖性,差异均<1%,在15 MV X射线能量下,无剂量率依赖性。MapCHECK 二维半导体矩阵在6 MV X射线和15 MV X射线能量下都显示了较强剂量率依赖性,响应差异在2%左右。结论 依据剂量率变化治疗技术的放疗加速器的剂量测量,需要对测量设备进行剂量率响应测试和分析,以保证日常刻度和剂量验证的精度。     

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.     

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.