A new evaluation of the decay data for 166Ho

The β--emitter 166Ho is of interest as a potential radiolabel for therapeutic medical applications. A new decay data evaluation for 166Ho has been performed using the Decay Data Evaluation Project (DDEP) methodology. New recommended values for the half-life, γ-ray emission probabilities, β-- branching ratios, and other relevant nuclear and atomic data are provided. This paper provides a summary of the evaluation; the complete set of recommended data tables and detailed evaluator comments are available at the DDEP website.

Absolute emission intensities of the gamma rays from the decay of 224Ra and 212Pb progenies and the half-life of the 212 Pb decay

Absolute gamma-ray emission intensities for 36 characteristic gamma rays from the decay of 224Ra, 212Pb, and their progeny were determined by measuring sources calibrated for activity by means of primary methods based on well-defined high-purity germanium (HPGe) detectors at both NIST and NPL. Results from the two laboratories agree with recent data evaluations, except for gamma rays with low emission intensities. The decay schemes have been re-balanced based on the new results. In addition, the half-life for 212Pb was measured using several HPGe detectors, ionization chambers, and a well-type NaI(Tl) detector.

Determination of the internal pair production branching ratio of 90Y

The National Institute of Standards and Technology (NIST) measured the internal pair production branching ratio of ⁹⁰Y using two sources and four high purity germanium (HPGe) detectors to detect the resulting annihilation radiation. The internal pair production branching ratio determined from these measurements, (32.0 ± 1.5) × 10^-6 (k = 1), agrees within 1 standard uncertainty with the recommended value of (32.6 ± 0.7) × 10^-6 (k = 1) from the DDEP database.

Standardization of 64Cu activity

The complex decay scheme that makes 64Cu promising as both an imaging and therapeutic agent in medicine also makes the absolute measurement of its activity challenging. The National Institute of Standards and Technology (NIST) has completed a primary activity standardization of a 64CuCl2 solution using the 4πβ(LS)-γ(NaI) live-timed anticoincidence (LTAC) counting method with a combined standard uncertainty of 0.51 %. Two liquid scintillation (LS) counting methods were employed for confirmatory measurements. Secondary measurements were made by high-purity germanium detectors, pressurized ionization chambers (IC), and a well-type NaI(Tl) counter. Agreement between the LTAC-based standard and standards from other laboratories was established via IC calibration factors. Poor agreement between methods and with theoretical IC responses may indicate a need for improved β+/- branching probabilities and a better treatment of β+/- spectra.

Results of an international comparison of activity measurements of 68Ge

An international key comparison, identifier CCRI(II)-K2.Ge-68, has been performed. The National Institute of Standards and Technology (NIST) served as the pilot laboratory, distributing aliquots of a 68Ge/68Ga solution. Results for the activity concentration, CA, of 68Ge at a reference date of 12h00 UTC 14 November 2014 were submitted by 17 laboratories, encompassing many variants of coincidence methods and liquid-scintillation counting methods. The first use of 4π(Cherenkov)β-γ coincidence and anticoincidence methods in an international comparison is reported. One participant reported results by secondary methods only. Two results, both utilizing pure liquid-scintillation methods, were identified as outliers. Evaluation using the Power-Moderated Mean method results in a proposed Comparison Reference Value (CRV) of 621.7(11)kBqg-1, based on 14 results. The degrees of equivalence and their associated uncertainties are evaluated for each participant. Several participants submitted 3.6mL ampoules to the BIPM to link the comparison to the International Reference System (SIR) which may lead to the evaluation of a Key Comparison Reference Value and associated degrees of equivalence.

Assessing the absolute quantitative accuracy of Positron Emission Tomography for Cu-64 using traceable calibrated phantoms

Using uniform cylindrical phantoms containing calibrated solutions of ¹⁸F and ⁶⁴Cu, we evaluated for the first time the accuracy with which the activity concentration of ⁶⁴Cu can be quantified on an absolute basis using Positron Emission Tomography (with X-ray Computed Tomography, PET-CT). The scanner was first calibrated for ¹⁸F using the manufacturer's calibration protocol and a phantom with an activity concentration value traceable to the U.S. National standard. By using a similarly calibrated ¹⁸F solution phantom, we were able to determine a correction factor that can be applied to the ⁶⁴Cu imaging data that gave a result that is consistent with 100% recovery with a combined standard uncertainty of 2%. We also demonstrate how a calibrated, solid phantom containing ⁶⁸Ge as a long-lived ¹⁸F surrogate can be used to monitor and transfer the correction factor to other studies.

Long-term stability of carrier-added (68)Ge standardized solutions

Tests for chemical stability were carried out on carrier-added (68)Ge solutions prepared and calibrated in 2007 and 2011 to evaluate the suitability of the specific composition as a potential Standard Reference Material. Massic count rates of the stored solutions were measured using a NaI(Tl) well counter before and after gravimetric transfers. The present activity concentration of the 2007 solution was also measured using live-timed anticoincidence counting (LTAC) and compared to the 2007 calibrated value. The well counter data indicated no change in massic count rate to within uncertainties for either solution. The LTAC measurements gave a difference of -0.49% in the activity concentration 2007 solution over 7 years. However, the uncertainty in the decay correction over that time, due to the uncertainty in the (68)Ge half-life, accounted for the majority (0.67% out of 0.83%) of the standard uncertainty on the activity concentration. The results indicate that these carrier-added solutions are stable with regard to potential activity losses over several half-lives of (68)Ge.

(Mis)use of (133)Ba as a calibration surrogate for (131)I in clinical activity calibrators

Using NIST-calibrated solutions of (131)Ba and (131)I in the 5mL NIST ampoule geometry, measurements were made in three NIST-maintained Capintec activity calibrators and the NIST Vinten 671 ionization chamber to evaluate the suitability of using (133)Ba as a calibration surrogate for (131)I. For the Capintec calibrators, the (133)Ba response was a factor of about 300% higher than that of the same amount of (131)I. For the Vinten 671, the Ba-133 response was about 7% higher than that of (131)I. These results demonstrate that (133)Ba is a poor surrogate for (131)I. New calibration factors for these radionuclides in the ampoule geometry for the Vinten 671 and Capintec activity calibrators were also determined.

Myelin-associated glycoprotein gene mutation causes Pelizaeus-Merzbacher disease-like disorder

Pelizaeus-Merzbacher disease is an X-linked hypomyelinating leukodystrophy caused by mutations or rearrangements in PLP1. It presents in infancy with nystagmus, jerky head movements, hypotonia and developmental delay evolving into spastic tetraplegia with optic atrophy and variable movement disorders. A clinically similar phenotype caused by recessive mutations in GJC2 is known as Pelizaeus-Merzbacher-like disease. Both genes encode proteins associated with myelin. We describe three siblings of a consanguineous family manifesting the typical infantile-onset Pelizaeus-Merzbacher disease-like phenotype slowly evolving into a form of complicated hereditary spastic paraplegia with mental retardation, dysarthria, optic atrophy and peripheral neuropathy in adulthood. Magnetic resonance imaging and spectroscopy were consistent with a demyelinating leukodystrophy. Using genetic linkage and exome sequencing, we identified a homozygous missense c.399C>G; p.S133R mutation in MAG. This gene, previously associated with hereditary spastic paraplegia, encodes myelin-associated glycoprotein, which is involved in myelin maintenance and glia-axon interaction. This mutation is predicted to destabilize the protein and affect its tertiary structure. Examination of the sural nerve biopsy sample obtained in childhood in the oldest sibling revealed complete absence of myelin-associated glycoprotein accompanied by ill-formed onion-bulb structures and a relatively thin myelin sheath of the affected axons. Immunofluorescence, cell surface labelling, biochemical analysis and mass spectrometry-based proteomics studies in a variety of cell types demonstrated a devastating effect of the mutation on post-translational processing, steady state expression and subcellular localization of myelin-associated glycoprotein. In contrast to the wild-type protein, the p.S133R mutant was retained in the endoplasmic reticulum and was subjected to endoplasmic reticulum-associated protein degradation by the proteasome. Our findings identify involvement of myelin-associated glycoprotein in this family with a disorder affecting the central and peripheral nervous system, and suggest that loss of the protein function is responsible for the unique clinical phenotype.

Fe/S protein assembly gene IBA57 mutation causes hereditary spastic paraplegia

OBJECTIVE

To present the clinical, molecular, and cell biological findings in a family with an autosomal recessive form of hereditary spastic paraplegia characterized by a combination of spastic paraplegia, optic atrophy, and peripheral neuropathy (SPOAN).

METHODS

We used a combination of whole-genome linkage analysis and exome sequencing to map the disease locus and to identify the responsible gene. To analyze the physiologic consequences of the disease, we used biochemical and cell biological methods.

RESULTS

Ten members of a highly consanguineous family manifested a childhood-onset SPOAN-like phenotype with slow progression into late adulthood. We mapped this disorder to a locus on chromosome 1q and identified a homozygous donor splice-site mutation in the IBA57 gene, previously implicated in 2 infants with lethal perinatal encephalomyopathy. This gene encodes the mitochondrial iron-sulfur (Fe/S) protein assembly factor IBA57. In addition to a severely decreased amount of normal IBA57 messenger RNA, a patient's cells expressed an aberrantly spliced messenger RNA with a premature stop codon. Lymphoblasts contained 10-fold-lower levels of wild-type, but no signs of truncated IBA57 protein. The decrease in functional IBA57 resulted in reduced levels and activities of several mitochondrial [4Fe-4S] proteins, including complexes I and II, while mitochondrial [2Fe-2S] proteins remained normal.

CONCLUSIONS

Our findings reinforce the suggested specific function of IBA57 in mitochondrial [4Fe-4S] protein maturation and provide additional evidence for its role in human disease. The less decreased IBA57 protein level in this family explains phenotypic differences compared with the previously described lethal encephalomyopathy with no functional IBA57.

A new NIST primary standardization of 18F

A new primary standardization of (18)F by NIST is reported. The standard is based on live-timed beta-gamma anticoincidence counting with confirmatory measurements by three other methods: (i) liquid scintillation (LS) counting using CIEMAT/NIST (3)H efficiency tracing; (ii) triple-to-double coincidence ratio (TDCR) counting; and (iii) NaI integral counting and HPGe γ-ray spectrometry. The results are reported as calibration factors for NIST-maintained ionization chambers (including some "dose calibrators"). The LS-based methods reveal evidence for cocktail instability for one LS cocktail. Using an ionization chamber to link this work with previous NIST results, the new value differs from the previous reports by about 4%, but appears to be in good agreement with the key comparison reference value (KCRV) of 2005.

Development of a calibration methodology for large-volume, solid ⁶⁸Ge phantoms for traceable measurements in positron emission tomography

We have developed a methodology to calibrate the (68)Ge activity concentration in large (9L) cylindrical epoxy phantoms in a way that is traceable to national standards. The method was tested on two prototype cylindrical phantoms that are being used in a clinical trial and gave (68)Ge activity concentration values with combined standard uncertainties of about 1.1%. Imaging data from the phantoms using a calibrated PET-CT scanner gave values consistent with the calibrated activity concentrations within experimental uncertainties.

Calibration of Traceable Solid Mock (131)I Phantoms Used in an International SPECT Image Quantification Comparison

The International Atomic Energy Agency (IAEA) has organized an international comparison to assess Single Photon Emission Computed Tomography (SPECT) image quantification capabilities in 12 countries. Iodine-131 was chosen as the radionuclide for the comparison because of its wide use around the world, but for logistical reasons solid (133)Ba sources were used as a long-lived surrogate for (131)I. For this study, we designed a set of solid cylindrical sources so that each site could have a set of phantoms (having nominal volumes of 2 mL, 4 mL, 6 mL, and 23 mL) with traceable activity calibrations so that the results could be properly compared. We also developed a technique using two different detection methods for individually calibrating the sources for (133)Ba activity based on a National standard. This methodology allows for the activity calibration of each (133)Ba source with a standard uncertainty on the activity of 1.4 % for the high-level 2-, 4-, and 6-mL sources and 1.7 % for the lower-level 23 mL cylinders. This level of uncertainty allows for these sources to be used for the intended comparison exercise, as well as in other SPECT image quantification studies.

A new primary standardization of 229Th

The National Institute of Standards and Technology (NIST) has certified a high-purity (229)Th Standard Reference Material as SRM 4328C, based on live-timed 4pialphabeta-gamma anticoincidence counting (LTAC) of the equilibrium solution. The LTAC system was optimized to minimize the uncertainty in the result due to the two short-lived ground-states present in the decay chain. Confirmatory measurements were carried out by four other methods. Furthermore, the present absolute activity and measured gamma-ray emission rates were combined to obtain gamma-ray emission probabilities.

63Ni, its half-life and standardization: revisited

Recent liquid scintillation (LS) measurements at the National Institute of Standards and Technology (NIST) and at the Laboratoire National Henri Becquerel (LNHB) on a standardized (63)Ni solution that has been tracked for nearly 40 years have resulted in several important findings: (i) a (63)Ni half-life value of 101.2 +/- 1.5 a has been determined with the present decay data. This value is consistent with a previous specific activity determination and with an earlier value from decay measurements; and it appears to be more satisfactory than a recent data evaluator's recommended value of 98.7 a. (ii) All solution standards of (63)Ni as disseminated by NIST for the past 38(+) years are internally consistent with past and recent standardizations. (iii) Primary LS standardizations of (63)Ni by the triple-to-double coincidence ratio (TDCR) method and by CIEMAT/NIST (3)H-standard efficiency tracing (CNET) appear to be comparable, although the latter methodology is believed to be inherently inferior. (iv) There is excellent measurement agreement between NIST and LNHB for (63)Ni primary standardizations.

Radionuclide metrology in the life sciences: Recent advances and future trends

This paper reviews activities in the field of radionuclide metrology applied to the life sciences between the years 2000 and 2005. The requirements for accuracy and consistency in making radioactivity measurements in radiation medicine, coupled with an increased awareness of the role of measurement standards in quality assurance programmes, has prompted a great deal of research in this area. During the past 5 years, particular emphasis has been on: (1) the development of primary standards for radionuclides, (2) development of secondary/transfer standards, (3) development of radionuclide standards for brachytherapy, and (4) inter-laboratory comparisons at the end-user level. Activities carried out by National Metrology Institutions in these areas are reviewed and a look at future trends is presented.

International guidance on the establishment of quality assurance programmes for radioactivity measurement in nuclear medicine

A new guidance document for the implementation of quality assurance (QA) programmes for nuclear medicine radioactivity measurement, produced by the International Atomic Energy Agency, is described. The proposed programme is based on the principles of ISO 17025 and will enable laboratories, particularly in developing countries, to provide consistent, safe and effective radioactivity measurement services to the nuclear medicine community.

Comparison of calculated spectra for the interaction of photons in a liquid scintillator. Example of 54Mn 835keV emission

The CIEMAT/NIST and TDCR methods in liquid scintillation counting, initially developed for the activity standardization of pure-beta radionuclides, have been extended to the standardization of electron capture and beta-gamma radionuclides. Both methods require the calculation of the energy spectrum absorbed by the liquid scintillator. For radionuclides emitting X-rays or gamma-rays, when the energy is greater than a few tens of keV the Compton interaction is important and the absorption is not total. In this case, the spectrum absorbed by the scintillator must be calculated using analytical or stochastic models. An illustration of this problem is the standardization of 54Mn, which is a radionuclide decaying by electron capture. The gamma transition, very weakly converted, leads to the emission of an 835 keV photon. The calculation of the detection efficiency of this radionuclide requires the calculation of the energy spectrum transferred to the scintillator after the absorption of the gamma ray and the associated probability of absorption. The validity of the method is thus dependent on the correct calculation of the energy transferred to the scintillator. In order to compare the calculation results obtained using various calculation tools, and to provide the metrology community with some information on the choice of these tools, the LS working group of the ICRM organised a comparison of the calculated absorbed spectra for the 835 keV photon of 54Mn. The result is the spectrum of the energy absorbed by the scintillator per emission of an 835 keV gamma ray. This exercise was proposed for a standard 20 ml LS glass vial and for LS cocktail volumes of 10 and 15 ml. The calculation was done for two different cocktails: toluene and a widely used commercial cocktail, Ultima Gold. The paper describes the results obtained by nine participants using a total of 12 calculation codes.

Monte Carlo calculations of spectra and interaction probabilities for photons in liquid scintillators for use in the standardization of radionuclides

The probabilities for the interaction of photons, Pint, in three different liquid scintillators (toluene, Ultima Gold, and Hionic Fluor) were calculated as a function of energy for three different geometries using the Monte Carlo codes PENELOPE and EGSnrcMP. The agreement in Pint values between the two codes in all of the geometries and scintillators for energies above 100 keV was within +/-0.5%. The agreement was better than +/-4% for energies between 20 and 100 keV. No substantive differences in the normalized absorbed energy spectra were observed for initial photon energies in the range of 20-1000 keV. The influence of scintillator composition on the simulation results was investigated in the context of a possible conflict in the published composition data for Hionic Fluor. A difference in interaction probability of about 27% was observed at 20 keV when an alternate composition having lower amounts of S and P is used. At 100 keV, the differences were within the calculational uncertainties.

Experimental determination of calibration settings for plastic syringes containing solutions of 90Y using commercial radionuclide calibrators

Calibration factors for several makes (Capintec, AtomLab, and PTW) and models of commercially available re-entrant ionization chambers ("dose calibrators") were determined for solutions of 90Y using 10 ml plastic syringes that are used in the administration of the recently approved radioimmunotherapy drug Zevalin. Effects of filling volume on chamber response were studied for the anticipated prescription volume range of 3-9 ml in those syringes. A series of syringes were prepared over that range with accurately known volumes using a "standard" solution containing 90YCl3 in 1 moll(-1) HCl and approximately 50 microg of nonradioactive YCl3 per gram of solution. The respective calibration factors for the Capintec and AtomLab chambers required to give the correct activity reading were found to be "55+/-2 x 10" and 393+/-6, where the uncertainties are expanded (k = 2) uncertainties. The results indicated no significant effect of filling volume on the calibration setting to within the uncertainty on the activity measurement for all but the PTW chamber. In that case, a variation of about 5% was observed over the filling range. A direct comparison was also made between the actual 90Y Zevalin drug and the "standard" solution, with no differences in determined calibration settings observed within the uncertainties on the activity calibration.

Construction and implementation of the NIST triple-to-double coincidence ratio (TDCR) spectrometer

A description of the triple-to-double coincidence ratio (TDCR) spectrometer recently constructed at NIST and results of tests to assess its operating characteristics are presented. Measured activities for previously calibrated solutions of 3H (tritiated water) and 63Ni agreed with certified activity values to within 0.04% and 0.2%, respectively. Agreement between measurements of solutions containing 90Y using the TDCR and the CIEMAT/NIST efficiency tracing method was achieved to within 0.7% and within the respective experimental uncertainties. A difference of 5.3% was observed between TDCR measurements and the certified massic activity value for a calibrated solution containing 54Mn, but because of the large uncertainty on the TDCR result (3.6% relative standard uncertainty), the values agree to within the experimental uncertainties. Agreement to within experimental uncertainties (2%) for 103Pd was achieved with the TDCR and CIEMAT/NIST methods, but not between TDCR and isothermal calorimetric measurements. TDCR results for a 204Tl solution were in excellent agreement (maximum difference of 0.2% with any other method, with combined standard uncertainty of 0.45% on TDCR result) with results obtained using three other methods.

Establishment of transfer standard for holmium-166-DOTMP

The measurement of 166Ho, both as a chloride solution and as [166Ho]-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylenephoonic acid (DOTMP), was examined for four models of radionuclide calibrators: Capintec CRC-35R (two chambers), Capintec 712MX, AtomLab 100 (two chambers), and a Capintec CRC-12. Holmium-166 chloride was measured as 16 ml in 20-ml glass dose vials. Diagnostic imaging level [166Ho]DOTMP solutions, nominally 400 MBqg(-1), were measured as 12 ml in 20-ml dose vials. Finally, therapeutic level [166Ho]DOTMP solutions, nominally 9GBqg(-1), were measured as aliquots of 100-500 microl in sealed plastic vials of 10-ml saline. Single calibration factors for each instrument manufacturer are recommended for 12-16-ml of either solution in 20-ml glass dose vials, (673+/-9) x 10 and 72.7+/-0.7, for the Capintec and AtomLab models, respectively. Calibration factors recommended for the therapeutic dose geometry are (706+/-6) x 10 and 68.7+/-1.3, for the Capintec and AtomLab models, respectively. The calibration factors recommended for an NIST 5-ml ampoule are (686+/-5) x 10 and 70.9+/-0.4 for the Capintec and AtomLab models, respectively.

Experimental investigation of dose calibrator response for 125I brachytherapy solutions contained in 5 mL plastic syringes and 2 mL conical glass v-vials as a function of filling mass

The effect of measurement geometry on the determination of the activity of solutions containing 125I for use in brachytherapy applications has been investigated for 5 mL plastic syringes and 2 mL conical glass dose vials as a function of filling mass. New dial settings for the syringes over a filling mass range of 1 to 3 g have been determined to be 497+/-8 and 469+/-8 (expanded, k = 2, uncertainties) for the NIST Capintec CRC- 12 and Capintec CRC-35R, respectively, with any effect due to the filling mass lying within the uncertainty in the activity calibration. A filling mass effect was observed in the dose vials, causing a 10.5% reduction in the chamber response from a 2 g filling mass to 1 g. Dial settings at 2 g were experimentally found to be 143+/-2 and 135+/-2 (expanded uncertainties) for the NIST Capintec CRC-12 and Capintec CRC-35R, respectively. The appropriate dial settings for the same vials with a 1 g filling mass were found to be 120+/-2 and 114+/-2 for CRC-12 and CRC-35R, respectively. Differences of up to +/-45% in the activity determination were observed between values obtained with the manufacturer's recommended setting and the settings obtained experimentally for each specific geometry. Calibration factors were also determined for a Vinten 671 Radionuclide Calibrator, giving values of 0.226+/-0.009 pA x MBq(-1) and 0.231+/-0.004 pA x MBq(-1) (expanded uncertainties), respectively, for the 1 and 2 g dispensings. This study demonstrates that experimentally determined calibration factors for the exact measurement geometry are necessary when measuring radionuclides in configurations other than the manufacturer's standard geometry, especially when nuclides that emit low-energy radiations are involved.

A dual-compensated cryogenic microcalorimeter for radioactivity standardizations

Efforts are underway by our laboratory to develop a microcalorimeter that can be routinely used for radioactivity standardizations of nuclides that decay by pure beta-emission or by low-Z electron capture. The prototype calorimeter consists of a cryostat with two temperature-controlled stages and a base stage that are operated at nominal temperatures of 8 K. A unique aspect of the calorimeter's design is the ability to repeatedly engage and disengage the radioactive source from the second stage heat path using a magnetically activated elevator. The measurement of the total power from a radioactive sample is obtained from the difference in the second stage power with and without the source in place. As a result of extensive performance evaluations using 90Sr-90Y and 32P brachytherapy seeds as well as with an internal calibration heater, many initial design flaws have been identified and are being addressed.

The standardization of 177Lu by 4pibeta liquid scintillation spectrometry with 3H-standard efficiency tracing

Solutions containing the potential radiotherapy radionuclide 177Lu have been standardized at the National Institute of Standards and Technology (NIST) by 4pibeta liquid scintillation (LS) counting with 3H-standard efficiency tracing using the CIEMAT/NIST method. Confirmatory measurements were made with 4pi NaI(TI) gamma-ray spectrometry. Activity determinations were made on 4 solutions over the course of 10 months with an expanded (k = 2) uncertainty on the activity of 0.8%. Half-life measurements were carried out using the NIST "4pi" gamma ionization chamber (IC) and LS counting and gave a new value of 6.65+/-0.01 d, which is shorter than the current ENSDF-recommended value by 1.3%. Impurity analyses were performed by high-purity germanium (HPGe) gamma-ray spectrometry and indicated only the presence of 177mLu at a level of 0.02% that of the 177Lu as of the respective reference times for the four solutions. Calibration factors for the NIST IC and Vinten 671 ionization chambers were developed, as were dial settings for the NIST-maintained Capintec CRC-12.

Radioassays and experimental evaluation of dose calibrator settings for 18F

The positron emitter 18F continues to be one of the most important imaging radionuclides in diagnostic nuclear medicine. Assays of radiopharmaceuticals containing this nuclide are often performed in the clinic using commercial reentrant ionization chambers, or "dose calibrators". Meaningful quantitative clinical studies require accurate knowledge of the injected activity which requires proper calibration of these instruments. Radioassays were performed at the National Institute of Standards and Technology (NIST) on a solution of 18F produced at the National Institutes of Health (NIH) using 4pibeta liquid scintillation (fS) counting with 3H-standard efficiency tracing. Cocktails containing water fractions of approximately 0.9 and 9% (both as saline) were used. The massic activity values were measured to be 2.52+/-0.06 and 2.50+/-0.03 MBq g(-1), respectively, for the 0.9 and 9% water cocktails as of the reference time. The uncertainties on the activity measurements are expanded (k = 2) uncertainties. The largest uncertainty component was found to be the repeatability on a single LS source, with the cocktails containing 0.9% water fraction exhibiting a larger variability by nearly a factor of two. Reproducibility between LS cocktails with the same water fraction was also found to be a large uncertainty component, but with a value less than half that due to measurement repeatability. Radionuclidic impurities consisted of 48V and 46Sc, at levels of 0.11+/-0.08% (expanded uncertainties) and approximately 2 x 10(-3)% (upper limit) relative to the activity of the 18F, as of the reference time. Dose calibrator dial settings for measuring solutions of 18F were experimentally determined for Capintec CRC-12 and CRC-35R dose calibrators in three measurement geometries: a 5-ml standard NIST ampoule (two ampoules measured), a 12-ml plastic syringe containing 9 ml of solution and a 10-ml Mallinckrodt molded dose vial filled with 5 ml of solution. The experimental dial settings (and the corresponding expanded uncertainties) for these geometries were found to be 477+/-7, 474+/-6, 482+/-6 and 463+/-7 for the two ampoules, the syringe and the dose vial, respectively, in the CRC-12. The dial settings determined for the CRC-35R were 472+/-7, 470+/-7, 464+/-6 and 456+/-6 for the two ampoules, the syringe, and the dose vial, respectively. The uncertainties in the dial settings are expanded uncertainties. Comparisons between the empirically determined dial settings and the manufacturer's recommended setting of "439" indicate that use of the manufacturer's setting overestimates the activity by between 3 and 6%, depending upon the geometry used.

Experimental determination of dose calibrator settings and study of associated volume dependence in v-vials for rhenium-186 perrhenate solution sources

OBJECTIVE

Accurate activity measurements of glass conical v-vials are only possible if dose calibrator dial settings are experimentally determined for the specific vial and volume range over which the measurements of a particular radionuclide is to be made. V-vials are used to transport and store unit doses of radiopharmaceuticals containing high-energy beta-emitters, such as 186Re. We have determined the correct dose calibrator dial settings for measuring 186Re in 3-mL glass conical v-vials from 2 manufacturers.

METHODS

The 186Re solutions used were calibrated for radioactivity content at the National Institute of Standards and Technology (NIST) using liquid scintillation counting with 3H-standard efficiency with a maximum expanded (k = 2) uncertainty of 1.2% on the activity. Volumes of the solutions then were accurately dispensed into a set of v-vials from each of the 2 manufacturers and assayed in the dose calibrator maintained at NIST.

RESULTS

For filling volumes above 1 mL, the dose calibrator response was found to be constant for both of the vials studied, enabling a single dial setting to be used for each vial type. The expanded uncertainties on the activity from uncertainty in the dial setting in that volume range were 0.4%-0.7%. Variability in vial construction contributed another 0.2%-0.3% in the uncertainty in the activity determination.

CONCLUSION

These studies indicate a strong volume dependence on the response of the dose calibrator and highlight the need for experimental verification of dose calibrator settings for nonstandard geometries.

Well-ionization chamber response relative to NIST air-kerma strength standard for prostate brachytherapy seeds

The response of well-ionization chambers to the emissions of 103Pd and 125I radioactive seed sources used in prostate cancer brachytherapy has been measured. Calibration factors relating chamber response (current or dial setting) to measured air-kerma strength have been determined for seeds from nine manufacturers, each with different designs. Variations in well-ionization chamber response relative to measured air-kerma strength have been observed because of differences in the emitted energy spectrum due to both the radionuclide support material (125I seeds) and the mass ratio of 103Pd to 102Pd (103Pd seeds). Obtaining accurate results from quality assurance measurements using well-ionization chambers at a therapy clinic requires knowledge of such differences in chamber response as a function of seed design.

Experimental determinations of commercial 'dose calibrator' settings for nuclides used in nuclear medicine

Commercial reentrant ionization chambers ('dose calibrators') have become the de facto standard instrument for making radioactivity measurements in the radiopharmaceutical industry. Accurate activity measurements, however, depend on the application of the correct calibration factor (dial setting) for the radionuclide in the exact measurement geometry for that particular instrument. Two methods are described for the experimental determination of dial settings for the Capintec CRC-12 dose calibrator. Using these methods, we have determined new calibration factors for 18F, 62Cu, 186Re and 188Re in several geometries. These new factors are presented, along with previously determined settings, for nuclides commonly used in nuclear medicine. The differences between the calibration factors for the different geometries, as well as the differences between the manufacturer's recommended setting underscore the need for empirically determined dose calibrator settings.

The standardization of 62Cu and experimental determinations of dose calibrator settings for generator-produced 62CuPTSM

The Positron Emission Tomography agent 62Cu Pyruvaldehyde Bis(N4-methyl)thiosemicarbazone (PTSM) has been standardized by the National Institute of Standards and Technology (NIST) using 4 pi beta liquid scintillation counting with 3H-standard efficiency tracing. Using a measurement model developed at NIST for the determination of experimental dose calibrator dial settings for short-lived radionuclides, the correct dial settings for 62CuPTSM in a clinically useful geometry (a 35 ml plastic syringe containing 33 ml of solution) as well as the standard NIST 5 ml glass ampoule have been elucidated. This measurement model is fully described, as is the treatment of associated uncertainties. The correct instrument settings for the NIST Capintec CRC-12 re-entrant ionization chamber ('dose calibrator') were found to be '499 +/- 6' and '489 +/- 8', respectively, for the syringe and ampoule geometries. The stated uncertainties are expanded (k = 2) uncertainties. The expanded uncertainties in the measured activity arising from these new dial settings are +/- 0.8% and +/- 1.4%, respectively, for the syringe and ampoule. The measured activities using these settings are lower than those obtained from the manufacture's recommended setting of '448' by 9.7% for the syringe and 7.7% for the ampoule, and underscore the need for experimental verification of the dose calibrator settings for each radionuclide and each geometry used in the clinic.

A new experimental determination of the dose calibrator setting for 188Re

Accurate activity measurements of radionuclides using commercial dose calibrators requires that the correct dial setting (or calibration factor) be applied. The dose calibrator setting for the medical radionuclide 188Re (as 188ReO4-) has been determined experimentally using solution sources prepared and calibrated at the National Institute of Standards and Technology (NIST).

METHODS

The specific activity of two sources (in units of MBq/g) in the standard 5-mL NIST ampoule and in a 5-mL SoloPak dose vial were calibrated using 4pibeta liquid scintillation counting with 3H-standard efficiency tracing and gamma-ray/bremmstrahlung counting in the NIST "4pi" gamma ionization chamber on gravimetrically related sources.

RESULTS

The newly determined settings for the NIST Capintec CRC-12 dose calibrator are (631+/-4) x 10 and (621+/-3) x 10 for the respective ampoule and dose vial geometries with an expanded (at a presumed 95% confidence level) uncertainty of 0.4%-0.5% in the activity determination. The setting for the dose vial geometry was independently confirmed using a Capintec CRC-15R at Cedars-Sinai Medical Center using sources calibrated against a NIST standard.

CONCLUSION

These new settings result in activity readings 28%-30% lower than those obtained using the previously recommended setting of 496 x 10. This discrepancy most likely results from underestimating the total radiation yield from 188Re decay when calculating the dose calibrator response. This study emphasizes the need for experimental determinations of dose calibrator settings in the geometry in which the measurements will be performed.

Determination of a calibration factor for the nondestructive assay of Guidant 32P brachytherapy sources

A calibration factor ('dial setting') for the nondestructive assay of Guidant TiNi-encapsulated 32P intravascular brachytherapy wire sources has been determined for measurements with the Capintec CRC-12 (sic. 'dose calibrator') ionization chamber. The calibration factor was derived from ionization current measurements with the CRC-12 followed by very quantitative, destructive assays of the 32P content in two sources.

The standardization of the potential bone palliation radiopharmaceutical 117mSn(+4)DTPA

Solutions containing the potential bone pain palliation radionuclide 117mSn, in chloride form and as a diethylenetriaminepentaacetate (DTPA) complex, have been standardized by 4 pi beta liquid scintillation (LS) spectrometry and 4 pi gamma-ray spectrometry. Massic activities of the stock solutions were measured in order to determine dose calibrator settings for the solutions using commercial dose calibrators. Excellent agreement in the measurement of solution massic activity between the two techniques was achieved. The massic activity of 117mSnCl4 stock solution was found to be 38.62 +/- 0.23 MBq g-1 and 38.81 +/- 0.94 MBq g-1 with LS spectrometry and 4 pi gamma-ray spectrometry respectively. The respective values of the massic activity of the 117mSnDTPA stock solution with LS spectrometry and 4 pi gamma-ray spectrometry were 39.35 +/- 0.23 MBq g-1 and 39.70 +/- 0.96 MBq g-1. Impurities were analyzed in several solutions and found to have emission rates on the order of 10(-4) to 10(-6) of the rate of the 117mSn emission at the end-of-bombardment. The largest impurities came from 113Sn and 125Sn, the activation products of isotopic impurities present in the 117Sn target. The relative proportions of the various impurities were found to be highly dependent upon the source of 117Sn target material. The implications of choice of half-life used in the decay correction of 117mSn are discussed.

National radioactivity standards for beta-emitting radionuclides used in intravascular brachytherapy

The uses of beta-particle emitting radionuclides in therapeutic medicine are rapidly expanding. To ensure the accurate assays of these nuclides prior to administration, radioactivity standards are needed. The National Institute of Standards and Technology (NIST), the national metrological standards laboratory for the United States, uses high-efficiency liquid scintillation counting to standardize solutions of such beta emitters, including 32P, 90Sr/90Y, and 188Re. Additional measurements are made on radionuclidic impurities, half lives, and other decay-scheme parameters (such as branching decay ratios or gamma-ray abundances) using HPGe detectors and reentrant ionization chambers. Following such measurements at NIST, standards are disseminated in three ways: Standard Reference Materials (SRMs), calibrations for source manufacturers, and calibration factors for commercial instruments. Uncertainties in the activity calibrations for these nuclides are of the order of +/-0.5% (at approximately 1-standard deviation confidence intervals).

A Compendium on the NIST Radionuclidic Assays of the Massic Activity of 63Ni and 55Fe Solutions Used for an International Intercomparison of Liquid Scintillation Spectrometry Techniques

The National Institute of Standards and Technology recently participated in an international measurement intercomparison for ⁶³Ni and ⁵⁵Fe, which was conducted amongst principal national radionuclidic metrology laboratories. The intercomparison was sponsored by EUROMET, and was primarily intended to evaluate the capabilities of liquid scintillation (LS) spectrometry techniques for standardizing nuclides that decay by low-energy β-emission (like ⁶³Ni) and by low-Z (atomic number) electron capture (like ⁵⁵Fe). The intercomparison findings exhibit a very good agreement for ⁶³Ni amongst the various participating laboratories, including that for NIST, which suggests that the presently invoked LS methodologies are very capable of providing internationally-compatible standardizations for low-energy β-emitters. The results for ⁵⁵Fe are in considerably poorer agreement, and demonstrated the existence of several unresolved problems. It has thus become apparent that there is a need for the various international laboratories to conduct rigorous, systematic evaluations of their LS capabilities in assaying radionuclides that decay by low-Z electron capture.

Standardization of 63Ni by 4πβ Liquid Scintillation Spectrometry With 3H-Standard Efficiency Tracing

The low energy (E_βmax = 66.945 keV ± 0.004 keV) β-emitter ⁶³Ni has become increasingly important in the field of radionuclidic metrology. In addition to having a low β-endpoint energy, the relatively long half-life (101.1 a ± 1.4 a) makes it an appealing standard for such applications. This paper describes the recent preparation and calibration of a new solution Standard Reference Material of ⁶³Ni, SRM 4226C, released by the National Institute of Standards and Technology. The massic activity C_A for these standards was determined using 4πβ liquid scintillation (LS) spectrometry with ³H-standard efficiency tracing using the CIEMAT/NIST method, and is certified as 50.53 kBq ·g^-1 ± 0.46 Bq · g^-1 at the reference time of 1200 EST August 15, 1995. The uncertainty given is the expanded (coverage factor k = 2 and thus a 2 standard deviation estimate) uncertainty based on the evaluation of 28 different uncertainty components. These components were evaluated on the basis of an exhaustive number (976) of LS counting measurements investigating over 15 variables. Through the study of these variables it was found that LS cocktail water mass fraction and ion concentration play important roles in cocktail stability and consistency of counting results. The results of all of these experiments are discussed.

Comparison of the French and U.S. national 3H (tritiated H2O) standards by 4 pi beta liquid scintillation spectrometry

Radioactivity standards of tritiated water (3H2O)-disseminated by the Laboratoire Primaire des Rayonnements Ionisants (LPRI) and the National Institute of Standards and Technology (NIST), the national radionuclidic metrology and standardization laboratories of France and U.S.A., respectively-have been intercompared by liquid scintillation (LS) spectrometry. The ratio of the certified massic activities for the two standards was compared to that obtained from direct measurements on matched sets of LS cocktails prepared from the standards. Seven experimental trials (involving a total of 21 counting sources for each standard) were performed for the comparison. The trials were performed under a wide range of experimental conditions, including use of two different LS spectrometers and three series of LS cocktail compositions (with systematically varied 3H detection efficiencies). The results exhibited an apparent mean disagreement between standards of < 0.4% on a relative basis. For contrast, the relative combined standard uncertainty on the massic activity ratio for the two standards, as obtained from their respective certified uncertainty assessments, is about 0.7%.

Primary cysts of the mastoid process

Two patients with primary mastoid cysts are presented. These examples stress the importance of regular follow-up of patients with chronic serous otitis media so that the complications or associated abnormalities may be treated early prior to extensive destructive erosion. The formation of these cysts may be due to changes brought about by chronic serous otitis media, or they may be of congenital origin with middle ear effusion as a sequela.