NIST Traceable Calibration Research Articles

Comparison of gamma-ray dosimeters in a field study in the Chernobyl exclusion zone

ABSTRACT Five different models of dosimeters were compared in the Chernobyl Inner Exclusion Zone by measuring gamma radiation in 12 locations. We used an instrument made by Mirion Technologies, Inc., as the reference, since that instrument had an NIST-traceable calibration. Two models of dosimeters gave radiation values similar to the Mirion at all levels of radiation encountered. Two other models gave similar values to the Mirion instrument at low radiation levels but not at higher radiation levels encountered. These results offer a caution regarding the accuracy of inexpensive commercially available radiation instruments. Implications: Comparison of gamma ray dosimeters in the Chernobyl Exclusion Zone showed varying agreement with an NIST-calibrated instrument, as well as varying linearity of response to ambient radiation. These results suggest caution regarding accuracy of inexpensive dosimeters. However, dosimeters used that were manufactured in Ukraine exhibited good agreement with the reference instrument.

Tc-HDP quantitative SPECT/CT in transthyretin cardiac amyloid and the development of a reference interval for myocardial uptake in the non-affected population

Background99mTechnetium-HDP (HDP) bone scans differentiate transthyretin (ATTR) cardiac amyloid from other infiltrative myocardial diseases. These scans are not quantitative and are assessed by comparing myocardial uptake to bone. This study examined whether quantitative HDP SPECT/CT can discriminate individuals with cardiac ATTR from the population without this disease.MethodsHDP thoracic xSPECT/CT QUANT (xQUANT) was performed in 29 patients: ATTR cardiac amyloid (n = 6); AL cardiac amyloid (n = 1); other infiltrative myocardial disease (n = 4); no known infiltrative cardiac disease (n = 18). SUVmax measured within volumes of interest for whole heart, ascending aorta blood pool, and specific bones. HDP myocardial uptake calculated as whole heart minus blood pool.ResultsThe cardiac ATTR group had greater HDP myocardial uptake than those with no known infiltrative disease (p = 0.002). AL and other myocardial diseases had uptake indistinguishable from the group with no known infiltrative cardiac disease. The SUVmaxima were sufficiently similar between individuals without cardiac ATTR that a 99% reference interval for HDP uptake could be calculated, providing an upper limit cut point of SUVmax 1.2. Individuals with cardiac ATTR had SUVmax well above this cut point.ConclusionQuantitative SPECT/CT can measure HDP myocardial uptake in individuals with normal hearts and those with cardiac ATTR without recourse to comparison with bone. It enables calculation of a reference interval for HDP myocardial uptake in the population without ATTR cardiac amyloid. Using this reference interval single individuals with cardiac ATTR can be accurately discriminated from the non-affected population. This technique uses a NIST traceable calibration source, potentially allowing development of multicentre clinical decision limits. Its role in disease management warrants further assessment.

Extrapolation Ionization Chamber Dosimetry of Fluorescent X-Ray Energies from 4.5 to 19.6 keV.

Characteristic X rays of energies less than approximately 20 keV are of interest in radiobiology and radiation oncology. There is evidence that these low-energy photons produce higher relative biological effectiveness (RBE) and lower oxygen enhancement ratio (OER) relative to higher energies. Lower energy X rays also offer the advantage of healthy tissue sparing beyond the target treatment depth. Electronic brachytherapy systems that can deliver characteristic and bremsstrahlung X rays of varying energy are in clinical use as well as under development. We performed low-energy extrapolation ionization chamber dosimetry using two methods: 1. the exposure-to-dose method; and 2. the Burlin theory method combined with the extrapolation chamber method of Klevenhagen. We investigated fluorescent X rays emitted from seven metals: titanium (Ti, Z = 22); chromium (Cr, Z = 24); iron (Fe, Z = 26); cobalt (Co, Z = 27); copper (Cu, Z = 29); zinc (Zn, Z = 30); and molybdenum (Mo, Z = 42). X rays were produced by irradiation of the metals with a 55 kVp, 45 mA silver anode spectrum. The data obtained were air kerma rate (cGy/min), and radiation dose rate (cGy/min) in phosphate-buffered saline (PBS) solution and water. Air kerma rates ranged from 3.55 ± 0.10 to 14.36 ± 0.39 cGy/min. Dose rates ranged from 3.85 ± 0.10 to 16.96 ± 0.46 cGy/min in PBS and 3.59 ± 0.10 to 16.06 ± 0.43 cGy/min in water. Dose-rate energy dependence of both models was examined by taking a ratio of measured to Monte Carlo calculated dose rates. Dosimetry method 1 exhibited a linear relationship across all energies with a slope of 0.0127 keV(-1) and R(2) of 0.9276. Method 2 exhibited a linear relationship across all energies with a slope of 0.0467 keV(-1) and R(2) of 0.9933. Method 1 or 2 may be used as a relative dosimetry system to derive dose rates to water by using a second reference ion chamber with a NIST-traceable calibration for the molybdenum spectrum.

TU‐D‐201‐03: Results of a Survey On the Implementation of the TG‐51 Protocol and Associated Addendum On Reference Dosimetry of External Beams

Purpose:The working group on the review and extension of the TG‐51 protocol (WGTG51) collected data from American Association of Physicists in Medicine (AAPM) members with respect to their current TG‐51 and associated addendum usage in the interest of considering future protocol addenda and guidance on reference dosimetry best practices. This study reports an overview of this survey on dosimetry of external beams.Methods:Fourteen survey questions were developed by WGTG51 and released in November 2015. The questions collected information on reference dosimetry, beam quality specification, and ancillary calibration equipment.Results:Of the 190 submissions completed worldwide (U.S. 70%), 83% were AAPM members. Of the respondents, 33.5% implemented the TG‐51 addendum, with the maximum calibration difference for any photon beam, with respect to the original TG‐51 protocol, being <1% for 97.4% of responses. One major finding is that 81.8% of respondents used the same cylindrical ionization chamber for photon and electron dosimetry, implying that many clinics are foregoing the use of parallel‐plate chambers. Other evidence suggests equivalent dosimetric results can be obtained with both cylindrical and parallel‐plate chambers in electron beams. This, combined with users comfort with cylindrical chambers for electrons will likely impact recommendations put forward in an upcoming electron beam addendum to the TG‐51 protocol. Data collected on ancillary equipment showed 58.2% (45.0%) of the thermometers (barometers) in use for beam calibration had NIST traceable calibration certificates, but 48.4% (42.7%) were never recalibrated.Conclusion:This survey provides a snapshot of TG‐51 external beam reference dosimetry practice in radiotherapy centers. Findings demonstrate the rapid take‐up of the TG‐51 photon beam addendum and raise issues for the WGTG51 to focus on going forward, including guidelines on ancillary equipment and the choice of chamber for electron beam dosimetry.

SU-F-BRA-07: Dosimetric Verification of the Valencia Skin Applicator Using Gafchromic EBT3 Film

Purpose: The Valencia applicators have recently been introduced for HDR treatment of small and shallow superficial skin lesions (< 20 mm diameter and 3-mm depth). Per AAPM TG 56, any HDR applicator internal dimensions must be verified prior to clinical use. However radiographic and tomographic imaging to validate the Valencia applicators is impractical due to the Tungsten alloy housing and flattening filter. In this study, we propose to use EBT3 film to indirectly confirm the physical integrity of the Valencia applicators. Methods: Treatment plans were created using the Oncentra MasterPlan TPS v4.5 for the H2 (20-mm dia.) and H3 (30-mm dia.) Valencia Applicators. A virtual CT phantom (2-mm slice thickness) was created with one source position in water. The published effective depth method was used for each applicator to delivery 500 cGy to a 3-mm depth using the TG-43 formalism. Film measurements (n=3) at 3-mm depth and vertical plane in solid water were performed for each applicator to verify the prescribed dose calculated by the TPS. Percent depth dose curves and off-axis profiles (phantom surface and 3-mm depth) were measured and compared to published data. Films were analyzed using an in-house written program and RIT113 v6 software. Film calibration was performed per TG-55 protocol using the Ir-192 source with NIST-traceable calibration. Results: The prescription absolute dose difference was 1% for the Valencia H2 applicator and 4% for the Valencia H3 applicator. The measured percent depth dose curves and off-axis dose profiles measured for the H2 and H2 Valencia applicators are in excellent agreement with the Granero et al. Monte Carlo results1. Conclusion: Gafchromic EBT3 film can be used to indirectly verify the internal components of special HDR skin applicators constructed from high Z materials.1Granero et al. “Design and evaluation of a HDR skin applicator with flattening filter”, Med. Phys. 35(2), 495–503, 2008

SU-F-BRA-08: An Investigation of Well-Chamber Responses for An Electronic Brachytherapy Source

Purpose: The aim of this study was to investigate the variation of well-type ionization chamber response between a Xoft Axxent™ electronic brachytherapy (EBT) source and a GE Oncoseed™ 6711 I-125 seed. Methods: A new EBT air-kerma standard has recently been introduced by the National Institute of Standards and Technology (NIST). Historically, the Axxent source strength has been based on a well chamber calibration from an I-125 brachytherapy source due to the lack of a primary standard. Xoft utilizes a calibration procedure that employs a GE 6711 seed calibration as a surrogate standard to represent the air-kerma strength of an Axxent source. This method is based on the premise that the energies of the two sources are similar and thus, a conversion factor would be a suitable interim solution until a NIST standard was established. For this investigation, a number of well chambers of the same model type and three different EBT sources were used to determine NIST-traceable calibration coefficients for both the GE 6711 seed and the Axxent source. The ratio of the two coefficients was analyzed for consistency and also to identify any possible correlations with chamber vintage or the sources themselves. Results: For all well chambers studied, the relative standard deviation of the ratio of calibration coefficients between the two standards is less than 1%. No specific trends were found with the well chamber vintage or between the three different EBT sources used. Conclusion: The variation of well chamber calibration coefficients between a Xoft Axxent™ EBT source versus a GE 6711 Oncoseed™ are consistent across well chamber vintage and between sources. The results of this investigation confirm the underlying assumptions and stability of the surrogate standard currently in use by Xoft, and establishes a migration path for future implementation of the new NIST air kerma standard. This research is supported in part by Xoft, a subsidiary of iCAD.

SU‐F‐19A‐06: Experimental Investigation of the Energy Dependence of TLD Sensitivity in Low‐Energy Photon Beams

Purpose:To measure the energy dependence of TLD sensitivity in lowenergy photon beams with equivalent mono‐energetic energy matching those of 103Pd, 125I and 131Cs brachytherapy sources.Methods:A Pantek DXT 300 x‐ray unit (Precision X‐ray, Branford, CT), with stable digital voltage control down to 20 kV, was used to establish three lowenergy photon beams with narrow energy spread and equivalent monoenergetic energies matching those of 103Pd, 125I and 131Cs brachytherapy sources. The low‐energy x‐ray beams and a reference 6 MV photon beam were calibrated according to the AAPM TG‐61 and TG‐51 protocols, respectively, using a parallel‐plate low‐energy chamber and a Farmer cylindrical chamber with NIST traceable calibration factors. The dose response of model TLD‐100 micro‐cubes (1×1×1 mm3) in each beam was measured for five different batches of TLDs (each contained approximately 100 TLDs) that have different histories of irradiation and usage. Relative absorbed dose sensitivity was determined as the quotient of the slope of dose response for a beam‐of‐interest to that of the reference beam.Results:Equivalent mono‐energetic photon energies of the low‐energy beams established for 103Pd, 125I and 131Cs sources were 20.5, 27.5, and 30.1 keV, respectively. Each beam exhibited narrow spectral spread with energyhomogeneity index close to 90%. The relative absorbed‐dose sensitivity was found to vary between different batches of TLD with maximum differences of up to 8%. The mean and standard deviation determined from the five TLD batches was 1.453 ± 0.026, 1.541 ± 0.035 and 1.529 ± 0.051 for the simulated 103P, 125I and 131Cs beams, respectively.Conclusion:Our measured relative absorbed‐dose sensitivities are greater than the historically measured value of 1.41. We find that the relative absorbed‐dose sensitivity of TLD in the 103P beam is approximately 5% lower than that of 125I and 131Cs beams. Comparison of our results with other studies will be presented.

Standardization of (68)Ge/(68)Ga Using Three Liquid Scintillation Counting Based Methods.

A solution containing 68Ge in equilibrium with its daughter, 68Ga, has been standardized for the first time at the National Institute of Standards and Technology (NIST) using 3 liquid scintillation-based techniques: live-timed 4πβ -γ anticoincidence (LTAC) counting, the Triple-to-Double Coincidence Ratio (TDCR) method, and 3H-standard efficiency tracing with the CIEMAT1/NIST (CNET) method. The LTAC technique is much less dependent on level scheme data and model-dependent parameters and was thus able to provide a reference activity concentration value for the master solution with a combined standard uncertainty of about 0.3 %. The other two methods gave activity concentration values with respective differences from the reference value of +1.2 % and −1.5 %, which were still within the experimental uncertainties. Measurements made on the NIST “4π”γ secondary standard ionization chamber allowed for the determination of calibration factors for that instrument, allowing future calibrations to be made for 68Ge/68Ga without the need for a primary measurement. The ability to produce standardized solutions of 68Ge presents opportunities for the development of a number of NIST-traceable calibration sources with very low (<1 %) relative standard uncertainties that can be used in diagnostic medical imaging.

Measurement of atmospheric NO2 by pulsed cavity ring‐down spectroscopy

We have constructed a pulsed cavity ring‐down spectrometer (CARDS) for simultaneous measurements of nitrogen dioxide (NO2), the nitrate radical (NO3), and dinitrogen pentoxide (N2O5) in the atmosphere. In this paper, we describe the development of the instrument to measure NO2 via its absorption at 532 nm. The NO2 detection channel was calibrated against a NIST traceable calibration standard as well as a photolysis‐chemiluminescence (P‐CL) NO2 detector. The absorption cross section of NO2 at 532 nm was determined to be (1.45 ± 0.06) × 10−19 cm2. The NO2 detection limit (1σ) for 1 s data is 40 pptv, and the instrument response is accurate within ±4% (1σ) under laboratory conditions. The linear dynamic range of the instrument has been verified from the detection limit to above 200 ppbv (r2 &gt; 99.99%). For field measurements it is necessary to correct the CARDS NO2 signal for absorption by ozone. Under ambient conditions we report 1 s NO2 CARDS data with total uncertainty ±(4%, 60 pptv + 0.4 × (pptv/ppbv) × O3) (1σ). The instrument was deployed in the field during the New England Air Quality Study–International Transport and Chemical Transformation on board the NOAA research vessel Ronald H. Brown in the summer of 2004 and in Boulder, Colorado, in the winter of 2005. In both campaigns, CARDS and P‐CL NO2 measurements were highly correlated (r2 &gt; 98%), indicating the absence of interfering gas phase absorbers at 532 nm other than ozone and the suitability of CARDS to measure NO2 in the troposphere.

Calibration of the photon component of 198Au stents

Purpose 198Au has promising characteristics for radioactive stent material, having properties as a mixed β-particle and γ emitter. Calibration of these radioactive stents is required to provide accurate clinical dosimetry. Methods and materials We have developed an electroplating technique to incorporate stable gold onto stents followed by activation to 198Au in the University of Wisconsin nuclear reactor. The calibration method is a modification of the NIST traceable, in-air calibration technique for high-dose-rate (HDR) 192Ir sources. Results The air-kerma strength of HDR and low-dose-rate (LDR) sources was measured for proof of principle and found to agree to within 3% of values obtained with other NIST traceable calibration techniques. The photon component of two 198Au radioactive stents was measured over a period of 3 days. Conclusion The air-kerma strength of HDR and LDR sources was measured for proof of principle and found to agree to within 3% of values obtained with other NIST traceable calibration techniques.

Calibration of Wafer Temperature to Nist Traceable Standards Using an Isothermal Cavity

ABSTRACTThe offset between Si temperature and indicated TC temperature when a thermocouple instrumented wafer is placed in a double side heated RTP system was measured by means of an isothermal cavity, formed by 2 parallel Si wafers mounted in the system. The lamp powers were controlled to get an equal wafer temperature, making the cavity isothermal. Inside the cavity, an R-type TC with a NIST traceable calibration was mounted. Outside the cavity, a standard type K 1530 SensArray thermocouple [1] was exposed to the typical conditions in an RTP system. The offset was measured as the difference between the indicated temperature of the standard 1530 TC and the temperature of the NIST traceable type R TC. The 3 sources of the offset are discussed and a typical correction factor is proposed.

Effect of viscosity on ultrasound wave reflection from a solid/liquid interface

A study of the simulated reflection of a wideband ultrasound shear wave from the solid/viscous fluid interface is presented. Various parameters affecting reflection factors including the material properties of the solid, fluid properties like density and viscosity, and the operating frequency are discussed. Simulated ultrasonic response waveforms are compared with the experimentally obtained data for NIST traceable calibration standards of viscosity. A good agreement was observed between the simulated and experimental waveforms at various viscosities and for different solid substrates.

Crack evaluation and growth during low cycle plastic fatigue—non-destructive techniques for detection

A study of the simulated reflection of a wideband ultrasound shear wave from the solid/viscous fluid interface is presented. Various parameters affecting reflection factors including the material properties of the solid, fluid properties like density and viscosity, and the operating frequency are discussed. Simulated ultrasonic response waveforms are compared with the experimentally obtained data for NIST traceable calibration standards of viscosity. A good agreement was observed between the simulated and experimental waveforms at various viscosities and for different solid substrates.