Commercial Ionization Chambers Research Articles

Accelerator-driven photon reference field for replacement of 137Cs γ-ray

A transition to a mechanical source for x-ray reference field is desired to replace high-activity Co-60 and Cs-137 gamma-ray sources for enhancing nuclear security and safety management. The National Metrology Institute of Japan (NMIJ) developed a new photon reference field using an electron beam from a linac with energy less than 1 MeV. The effective energy of the photon field is comparable with Cs-137 gamma-ray. The air kerma rate in the reference field was validated by comparing a commercial cavity ionization chamber to the NMIJ standard reference field. The calibration constants were consistent within 0.8% for every relative extended uncertainty (k = 2).

Measurement of radiation-induced current through a SiC Schottky barrier diode in a clinical carbon beam cancer therapy field

Real-time physical dose evaluation was performed using radiation-tolerant SiC Schottky barrier diodes (SBDs). The radiation-induced current (RIC) generated in the SBD was measured under clinical carbon beam irradiation. A SiC SBD semiconductor detector (epitaxial layer, 69 µm thick; impurity concentration, 9.1 × 1014 cm−3; electrodes, 1 mm2) was used. The SiC SBD was irradiated with carbon ions having energy of 290 MeV/n at a maximum flux of therapeutic intensity (1 × 109 particles per second) at the Gunma University Heavy Ion Medical Center. The transition of the RIC value at any depth point in the Bragg curve was evaluated by changing the thickness of a water tank placed in front of the SiC SBD. The RIC from the SiC SBD was obtained with an applied voltage of up to 100 V for each moderator thickness. The physical dose distribution was also measured using a commercial ionization chamber. The Bragg peak was determined from the RIC distribution, and the transition was similar to that observed using the commercial ionization chamber.

APMP/TCRI key comparison report of measurement of air kerma for medium-energy x-rays (APMP.RI(I)-K3.2013)

Main textThe APMP/TCRI Dosimetry Working Group initiated the APMP.RI(I)-K3.2013 key comparison of measurement of air kerma for medium-energy x-rays (100 kV to 250 kV); measurements took place between 2015 and 2017. In total, 11 institutes took part in the comparison, among which 8 were APMP member laboratories, two from AFRIMETS and one from SIM. Three commercial cavity ionization chambers were used as transfer standards circulated among the participants. All the participants established 100, 135, 180 and 250 kV x-ray beam qualities equivalent to those of the BIPM. The ARPANSA and the NMIJ were linking laboratories and two participants subsequently made comparisons with the BIPM. The 7 other participants' degrees of equivalence fell within 11 parts in 103 of unity, well within the relevant expanded uncertainty (k = 2) in all cases. Excluding the BARC, the results were within 5.7 parts in 103, well within the relevant combined standard uncertainty. These results indicated that the calibration capabilities of most laboratories were in very good agreement and there were no serious discrepancies between them. The effect of implementing the recommendations of ICRU Report 90, which occurred on 1 January 2018 (after this comparison), is discussed and we deduce that the effect on the degrees of equivalence is negligible. The increase in the uncertainty in Wair has been added to all laboratories so that the degrees of equivalence and the uncertainties published here are applicable to post-ICRU 90 measurement results.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

In-situ radon-in-water detection for high resolution submarine groundwater discharge assessment

Submarine groundwater discharge (SGD), including both land-based fresh groundwater that enters the ocean from coastal aquifers as well as recirculated seawater that is continuously recharged and discharged on the seabed, has been considered as an important component of the global water and biogenic element (e.g., nitrogen, phosphorus, silicon and carbon) sources and a significant pathway for material exchange at the land-sea interface of coastal ecosystems. Some researchers reported that SGD associated nutrient additions to coastal waters have caused unwanted ecological issues, including red tides, coastal acidification and hypoxia. Natural radon isotope (222Rn, t1/2 = 3.8 d) is an excellent tracer for studying SGD and other oceanographic processes including air-sea gas exchange, sediment-water diffusion, and earthquake prediction. However, the conventional radon measurement methods suffer many technical disadvantages. We consequently developed a convenient submersible radon determination approach (“OUC-Rn”) using a commercial pulsed ionization chamber (PIC) radon sensor and gas extraction membrane module to produce high precision and high resolution observations. We demonstrate the radon degassing efficiency of the membrane contactor is comparable to the shower-head type air-water exchanger but is independent of operating position. The radon measurement efficiency of the PIC is 2-fold higher than the RAD7 detector and is far less influenced by moisture. We successfully deployed the system in 2.5 meters water depth over a 100 hours period in an anthropogenic influenced bay. Based on our high temporal resolution observations, the SGD flux was estimated to be 0-43.0 cm/d (mean: 25.4 ± 14.5 cm/d). The SGD fluxes pattern plotted together with the tidal variations revealed that tidal pumping may be the main force driving seawater recirculation into aquifers and thus affecting nutrient, carbon and other dissolved matters dynamics in coastal regions.

Monte Carlo optimization and experimental validation of a prototype ionization chamber for accurate magnetic resonance image guided radiation therapy (MRgRT) daily output constancy measurements in solid phantoms.

To optimize the design, develop and test a prototype ionization chamber for accurate daily output constancy measurements in solid phantoms in clinical magnetic resonance-guided radiation therapy (MRgRT) radiotherapy beams. Up to 4% variations in response using commercial ionization chambers have been previously reported; the prototype ionization chamber developed here aims to minimize thesevariations. Monte Carlo simulations with the EGSnrc code system are used to optimize an ionization chamber design by increasing the thickness of a brass (high-density, nonferromagnetic, easy-to-machine) wall until results consistent with no air gap are produced for simulations with a 1.5 T and 0.35T magnetic field, with a 0.2mm air gap and varying the placement of the chamber model within the air gap. Based on the results of these simulations, prototype ionization chambers are manufactured and tested in conventional linac beams and in a 7MV Elekta Unity MR-linac. The chambers are rotated about their axes, both parallel and perpendicular to the 1.5T magnetic field, through 360º in a plastic phantom with measurements made at each cardinal angle. This reveals any variation in chamber response by varying the thickness of the air gap between the chamber and thephantom. Monte Carlo simulations demonstrate that the optimal thickness of the chamber wall to mitigate the effect of an asymmetric air gap between the chamber and the plastic phantom is 1.1mm of brass. With this thickness, the differences between simulations with and without an air gap and with asymmetric placement of the chamber within the air gap are less than 0.2%. A prototype chamber constructed with a 1.1mm brass wall thickness exhibits less than 0.3% variation in response when rotated about its axis in the plastic phantom in a beam from an MR-linac, independent of whether its axis is parallel or perpendicular to the magneticfield. The optimized ionization chamber design and validated prototype for accurate MR-linac daily output constancy measurements allows utilization of conventional phantoms and procedures in MRgRT systems. This can minimize disruption to clinical workflow for MR-linac quality assurancemeasurements.

APMP key comparison report of air kerma for 137Cs (APMP.RI(I)-K5)

Main textThe APMP/TCRI Dosimetry Working Group performed the APMP.RI(I)-K5 key comparison of the air kerma for 137Cs in 2014. Five national metrology institutes (NMIs) took part in the comparison. Two commercial ionization chambers were used as transfer instruments and circulated among the participants. The results showed that the maximum difference between the participants and the Bureau International des Poids et Mesures, evaluated using the comparison data of the linking laboratories of the Korea Research Institute of Standards and Science and the National Metrology Institute of Japan, was less than 0.5% within the expanded uncertainty. This comparison supports the equivalence of the calibration capabilities of the participating laboratories. The results predate the publication of ICRU report 90, therefore, the revision of the data reflecting the effects of the ICRU report 90 on the degrees of equivalences of the participant laboratories is presented in Appendix C.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Standardization of 223Ra by live-time anticoincidence counting and gamma-ray emission determination

From a commercial supplier, three independent lots of an aqueous solution containing 223Ra in equilibrium with its deteriorating progeny were considered for standardization, by using live-time anticoincidence counting (LTAC) in the Laboratório Nacional de Metrologia das Radiações Ionizantes (LNMRI) Brazil. The ionization chamber calibration factors were obtained using measurements of independent lots of 223Ra in LTAC considering the absolute method in order to evaluate the constancy and reproducibility of the standardization. The calibration factors of three high-performance well-type ionization chamber systems and four commercial well-type ionization chambers were determined. The LTAC results showed that lots 2 and 3 were 1069.88 and 1097.44 kBq/kg, with corresponding relative standard uncertainties 0.12% and 0.20%, respectively. The spectrometry method results of each lot value were classified as positive by ANOVA hypothesis testing. The emission probabilities relating to the X- and γ-rays in region from 81 to 830 keV ware determined.Hence, LNMRI can provide calibration services for 223Ra.

APMP key comparison of absorbed dose to water for 60Co, APMP.RI(I)-K4

Main textThe APMP/TCRI Dosimetry Working Group performed the APMP.RI(I)-K4 key comparison of absorbed dose to water for 60Co between 2009 and 2010. Fourteen standards laboratories took part in the comparison. Three commercial cavity ionization chambers were used as transfer instruments and circulated among the participants. Nearly all the measured calibration coefficients for each ionization chamber were within one standard uncertainty (as estimated by the laboratory) of the comparison mean for that chamber, indicating reasonable agreement amongst the participants. Three participants (ARPANSA, PTB and NMIJ) were used to link the results to the BIPM through the ongoing bilateral BIPM.RI(I)-K4 comparison. Through this link the degree of equivalence (the ratio to the BIPM Key Comparison Reference Value and its uncertainty) was calculated for each non-linking laboratory to show the calibration capabilities. For the 9 eligible participants, this degree of equivalent ratio fell within 3.3 parts in 103 of unity, well within the combined standard uncertainty in all cases.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/.The final report has been peer-reviewed and approved for publication by the CCRI, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Primary standardization of 224Ra activity by liquid scintillation counting

A standard for activity of 224Ra in secular equilibrium with its progeny has been developed, based on triple-to-double coincidence ratio (TDCR) liquid scintillation (LS) counting. The standard was confirmed by efficiency tracing and 4παβ(LS)-γ(NaI(Tl)) anticoincidence counting, as well as by 4πγ ionization chamber and NaI(Tl) measurements. Secondary standard ionization chambers were calibrated with an expanded uncertainty of 0.62% (k = 2). Calibration settings were also determined for a 5 mL flame-sealed ampoule on several commercial reentrant ionization chambers (dose calibrators).

Patient-Specific Energy Verify Using the Multilayer Acrylic Disk Detector for Proton Pencil Beam Scanning

The total dose distribution of the proton pencil beam scanning (PBS) is constructed by individually delivered Bragg peaks and its uniformity depends directly on the accuracy of the delivery (i.e., the beam energy and the spot location). This study is the major reason for verifying the layer-by-layer energy along the scan path of PBS and was newly constructed the acrylic-disk radiation sensor (ADRS) of 20-layers to compare efficiency with the commercial multilayer ionization chamber (MLIC). We analyzed the properties of a multilayer ADRS to evaluate that it is suitable for the new QA system, and to compare the measured patient-specific energy with those predicted by the treatment planning system (TPS). The final goal of this study is to suggest optimized systems and methods for quickly and accurately carry out existing QA processes in PBS proton therapy. The measurements were obtained from a clinical proton pencil beams (IBA Proton Therapy System-Proteus 235). The multilayer ADRS is a series of transmitter disk (BC-800) with 150 mm diameter surrounded by thin optical fiber (PGS-CD1001-13-E). This disk is an ultraviolet-transparent methyl methacrylate polymer and shows water equivalency (mass density of 1.19 gcm-3) and has many other advantages; easy processing, remote sensing, and no interference from electromagnetic fields. It is now improved the efficiency of the measurement by arranging the ADRS of 20-layers and fabricating black plastic cases that allow for flexibility in placement of the solid slabs phantom, including in the extra room. It is also designed with a 1 mm thick disk which is as thin as can be reliably manufactured for verifying the layer-by-layer energy. The new is a significant step forward towards high spatial resolution compared to the previous version, which was made with a thickness of 2 mm. We investigated the feasibility of the multilayer ADRS to verify the layer-by-layer energy along the scan path for patients treated with PBS in three different sites (prostate, head & neck, and lung). The inaccuracy effect of the beam energy creates an uneven distance between adjacent layers in depth and thus cause ripples in depth, particularly at the shallower part where the Bragg peak is distinctively narrow. To evaluate the patient-specific energy, results calculated by the TPS and measured by the multilayer ADRS showed significant differences in the integrated energy of each layer depending on the treatment site. Therefore, the patient-specific energy calculated by the TPS must be compared and verified by the measured value prior to treatment. In this study, we believe that the multilayer ADRS has considerable potential to verify the energy for PBS proton therapy system.

Implementation of planar proton minibeam radiation therapy using a pencil beam scanning system: A proof of concept study.

Proton minibeam radiation therapy (pMBRT) is an innovative approach that combines the advantages of minibeam radiation therapy with the more precise ballistics of protons to further reduce the side effects of radiation. One of the main challenges of this approach is the generation of very narrow proton pencil beams with an adequate dose-rate to treat patients within a reasonable treatment time (several minutes) in existing clinical facilities. The aim of this study was to demonstrate the feasibility of implementing pMBRT by combining the pencil beam scanning (PBS) technique with the use of multislit collimators. This proof of concept study of pMBRT with a clinical system is intended to guide upcoming biological experiments. Monte Carlo simulations (TOPAS v3.1.p2) were used to design a suitable multislit collimator to implement planar pMBRT for conventional pencil beam scanning settings. Dose distributions (depth-dose curves, lateral profiles, Peak-to-Valley Dose Ratio (PVDR) and dose-rates) for different proton beam energies were assessed by means of Monte Carlo simulations and experimental measurements in a water tank using commercial ionization chambers and a new p-type silicon diode, the IBA RAZOR. An analytical intensity-modulated dose calculation algorithm designed to optimize the weight of individual Bragg peaks composing the field was also developed and validated. Proton minibeams were then obtained using a brass multislit collimator with five slits measuring 2cm×400μm in width with a center-to-center distance of 4mm. The measured and calculated dose distributions (depth-dose curves and lateral profiles) showed a good agreement. Spread-out Bragg peaks (SOBP) and homogeneous dose distributions around the target were obtained by means of intensity modulation of Bragg peaks, while maintaining spatial fractionation at shallow depths. Mean dose-rates of 0.12 and 0.09Gy/s were obtained for one iso-energy layer and a SOBP conditions in the presence of multislit collimator. This study demonstrates the feasibility of implementing pMBRT on a PBS system. It also confirms the reliability of RAZOR detector for pMBRT dosimetry. This newly developed experimental methodology will support the design of future preclinical research with pMBRT.

An update on 'dose calibrator' settings for nuclides used in nuclear medicine.

Most clinical measurements of radioactivity, whether for therapeutic or imaging nuclides, rely on commercial re-entrant ionization chambers ('dose calibrators'). The National Institute of Standards and Technology (NIST) maintains a battery of representative calibrators and works to link calibration settings ('dial settings') to primary radioactivity standards. Here, we provide a summary of NIST-determined dial settings for 22 radionuclides. We collected previously published dial settings and determined some new ones using either the calibration curve method or the dialing-in approach. The dial settings with their uncertainties are collected in a comprehensive table. In general, current manufacturer-provided calibration settings give activities that agree with National Institute of Standards and Technology standards to within a few percent.

2D convolution kernels of ionization chambers used for photon-beam dosimetry in magnetic fields: the advantage of small over large chamber dimensions

This study aims at developing an optimization strategy for photon-beam dosimetry in magnetic fields using ionization chambers. Similar to the familiar case in the absence of a magnetic field, detectors should be selected under the criterion that their measured 2D signal profiles M(x,y) approximate the absorbed dose to water profiles D(x,y) as closely as possible. Since the conversion of D(x,y) into M(x,y) is known as the convolution with the ‘lateral dose response function’ K(x-ξ, y-η) of the detector, the ideal detector would be characterized by a vanishing magnetic field dependence of this convolution kernel (Looe et al 2017b Phys. Med. Biol. 62 5131–48). The idea of the present study is to find out, by Monte Carlo simulation of two commercial ionization chambers of different size, whether the smaller chamber dimensions would be instrumental to approach this aim.As typical examples, the lateral dose response functions in the presence and absence of a magnetic field have been Monte-Carlo modeled for the new commercial ionization chambers PTW 31021 (‘Semiflex 3D’, internal radius 2.4 mm) and PTW 31022 (‘PinPoint 3D’, internal radius 1.45 mm), which are both available with calibration factors. The Monte-Carlo model of the ionization chambers has been adjusted to account for the presence of the non-collecting part of the air volume near the guard ring.The Monte-Carlo results allow a comparison between the widths of the magnetic field dependent photon fluence response function KM(x-ξ, y-η) and of the lateral dose response function K(x-ξ, y-η) of the two chambers with the width of the dose deposition kernel KD(x-ξ, y-η). The simulated dose and chamber signal profiles show that in small photon fields and in the presence of a 1.5 T field the distortion of the chamber signal profile compared with the true dose profile is weakest for the smaller chamber. The dose responses of both chambers at large field size are shown to be altered by not more than 2% in magnetic fields up to 1.5 T for all three investigated chamber orientations.

A Complexity-Based Correction Factor for a Commercial 4-Dimensional Ionization Chamber Array System on IMRT Dosimetry Quality Assurance Measurements

A Complexity-Based Correction Factor for a Commercial 4-Dimensional Ionization Chamber Array System on IMRT Dosimetry Quality Assurance Measurements

Reference dosimetry in magnetic fields: formalism and ionization chamber correction factors.

Magnetic resonance imaging-guided radiotherapy (MRIgRT) provides superior soft-tissue contrast and real-time imaging compared with standard image-guided RT, which uses x-ray based imaging. Several groups are developing integrated MRIgRT machines. Reference dosimetry with these new machines requires accounting for the effects of the magnetic field on the response of the ionization chambers used for dose calibration. Here, the authors propose a formalism for reference dosimetry with integrated MRIgRT devices. The authors also examined the suitability of the TPR10 (20) and %dd(10)x beam quality specifiers in the presence of magnetic fields and calculated detector correction factors to account for the effects of the magnetic field for a range of detectors. The authors used full-head and point-source Monte Carlo models of an MR-linac along with detailed detector models of an Exradin A19, an NE2571, and several PTW Farmer chambers to calculate magnetic field correction factors for six commercial ionization chambers in three chamber configurations. Calculations of ionization chamber response (performed with geant4) were validated with specialized Fano cavity tests. %dd(10)x values, TPR10 (20) values, and Spencer-Attix water-to-air restricted stopping power ratios were also calculated. The results were further validated against measurements made with a preclinical functioning MR-linac. The TPR10 (20) was found to be insensitive to the presence of the magnetic field, whereas the relative change in %dd(10)x was 2.4% when a transverse 1.5 T field was applied. The parameters chosen for the ionization chamber calculations passed the Fano cavity test to within ∼0.1%. Magnetic field correction factors varied in magnitude with detector orientation with the smallest corrections found when the chamber was parallel to the magnetic field. Reference dosimetry can be performed with integrated MRIgRT devices by using magnetic field correction factors, but care must be taken with the choice of beam quality specifier and chamber orientation. The uncertainties achievable under this formalism should be similar to those of conventional formalisms, although this must be further quantified.

Pencil beam proton radiography using a multilayer ionization chamber

A pencil beam proton radiography (PR) method, using a commercial multilayer ionization chamber (MLIC) integrated with a treatment planning system (TPS) was developed.A Giraffe (IBA Dosimetry) MLIC (±0.5 mm accuracy) was used to obtain pencil beam PR by delivering spots uniformly positioned at a 5.0 mm distance in a 9 × 9 square of spots. PRs of an electron-density (with tissue-equivalent inserts) phantom and a head phantom were acquired. The integral depth dose (IDD) curves of the delivered spots were computed by the TPS in a volume of water simulating the MLIC, and virtually added to the CT at the exit side of the phantoms. For each spot, measured and calculated IDD were overlapped in order to compute a map of range errors. On the head-phantom, the maximum dose from PR acquisition was estimated. Additionally, on the head phantom the impact on the range errors map was estimated in case of a 1 mm position misalignment.In the electron-density phantom, range errors were within 1 mm in the soft-tissue rods, but greater in the dense-rod. In the head-phantom the range errors were −0.9 ± 2.7 mm on the whole map and within 1 mm in the brain area. On both phantoms greater errors were observed at inhomogeneity interfaces, due to sensitivity to small misalignment, and inaccurate TPS dose computation. The effect of the 1 mm misalignment was clearly visible on the range error map and produced an increased spread of range errors (−1.0 ± 3.8 mm on the whole map). The dose to the patient for such PR acquisitions would be acceptable as the maximum dose to the head phantom was <2cGyE.By the described 2D method, allowing to discriminate misalignments, range verification can be performed in selected areas to implement an in vivo quality assurance program.

Polarity correction factor for flattening filter free photon beams in several cylindrical ionization chambers.

In this study, we aimed to compare the polarity correction factor in ionization chambers for flattening filter free (FFF) photon beams and flattening filter (FF) beams. Measurements were performed with both 6 and 10 MV FFF and FF beams. Five commercial ionization chambers were evaluated: PTW TN30013; IBA Dosimetry CC01, CC04, and CC13; and Exradin A12S. Except for the CC01 ionization chamber, the other four chambers showed less than a 0.3% difference in the polarity effect between the FFF and the FF beams. The CC01 chamber showed a strong field-size-dependence, unlike the other chambers. The polarity effect for all chambers with FFF beams did not change with the dose rate. Except in the case of the CC01 chamber, the difference in the polarity effect between FFF and FF beams was not significant.

Clinical implementation of photon beam flatness measurements to verify beam quality

This work describes the replacement of Tissue Phantom Ratio (TPR) measurements with beam profile flatness measurements to determine photon beam quality during routine quality assurance (QA) measurements. To achieve this, a relationship was derived between the existing TPR15/5 energy metric and beam flatness, to provide baseline values and clinically relevant tolerances. The beam quality was varied around two nominal beam energy values for four matched Elekta linear accelerators (linacs) by varying the bending magnet currents and reoptimizing the beam. For each adjusted beam quality the TPR15/5 was measured using an ionization chamber and Solid Water phantom. Two metrics of beam flatness were evaluated using two identical commercial ionization chamber arrays. A linear relationship was found between TPR15/5 and both metrics of flatness, for both nominal energies and on all linacs. Baseline diagonal flatness (FDN) values were measured to be 103.0% (ranging from 102.5% to 103.8%) for 6 MV and 102.7% (ranging from 102.6% to 102.8%) for 10 MV across all four linacs. Clinically acceptable tolerances of ±2% for 6 MV, and ±2% for 10 MV, were derived to equate to the current TPR15/5 clinical tolerance of ±0.5%. Small variations in the baseline diagonal flatness values were observed between ionization chamber arrays; however, the rate of change of TPR15/5 with diagonal flatness was found to remain within experimental uncertainty. Measurements of beam flatness were shown to display an increased sensitivity to variations in the beam quality when compared to TPR measurements. This effect is amplified for higher nominal energy photons. The derivation of clinical baselines and associated tolerances has allowed this method to be incorporated into routine QA, streamlining the process whilst also increasing versatility. In addition, the effect of beam adjustment can be observed in real time, allowing increased practicality during corrective and preventive maintenance interventions.PACS number: 87.56.Fc

Characterization tests of a new parallel plate ionization chamber for use in electron beams

Abstract Linear accelerators with electron beams are used in several Brazilian hospitals. Consequently, there is an increasing demand for parallel-plate ionization chambers, to be utilized for dosimetry of electron beams. In Brazil, the commercial ionization chambers utilized are imported. The ionization chambers have usually a simple construction, using different materials and geometries. A homemade ionization chamber was developed to be used in electron beams of linear accelerator. The ionization chamber body is made of acrylic and the collecting electrode is painted with graphite powder mixed with nail polish. Several tests were applied, and the results showed values better than the limits established by the international recommendations, except for the polarity effect test, but the response of the developed ionization chamber, for this test, is similar in relation to the response of other commercial ionization chambers from the literature.

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.