Activity standardization by liquid scintillation counting and half-life measurements of

Theranostic Imaging of Yttrium-90

This paper overviews Yttrium-90 ((90)Y) as a theranostic and nuclear medicine imaging of (90)Y radioactivity with bremsstrahlung imaging and positron emission tomography. In addition, detection and optical imaging of (90)Y radioactivity using Cerenkov luminescence will also be reviewed. Methods and approaches for qualitative and quantitative (90)Y imaging will be briefly discussed. Although challenges remain for (90)Y imaging, continued clinical demand for predictive imaging response assessment and target/nontarget dosimetry will drive research and technical innovation to provide greater clinical utility of (90)Y as a theranostic agent.

Activity determination of (59)Fe

Iron-59 was measured in three commercial and two custom-built liquid scintillation counters. The counting efficiencies were determined using CIEMAT/NIST efficiency tracing and the triple-to-double coincidence ratio (TDCR) method, respectively. The efficiency computation for the TDCR method was realized by means of the MICELLE2 program, applying a stochastic model for the computation of electron emission spectra. The program was extended to make calculations of spectra originating from complex decay schemes possible. In addition, a new parameterization of electron stopping powers for 10 commercial liquid scintillation cocktails was included in the software. The activities determined with the two methods were in very good agreement; the relative standard uncertainty of the combined result was found to be 0.16%. It was used to calibrate a 4π ionization chamber at PTB for future calibrations of this isotope which is used for investigations of iron metabolism. A standardized solution was submitted to the Bureau International des Poids et Mesures (BIPM) to be measured in the ionization chambers of the International Reference System (SIR) for comparison purposes. The liquid scintillation samples were also measured in a new portable TDCR system with three channel photomultipliers. Although this system has a much lower counting efficiency, the activity was in satisfactory agreement with the conventional TDCR system. The usage of the portable TDCR system, thus, provides an important test of the free parameter model.

Comparison of yttrium-90 quantitative imaging by TOF and non-TOF PET in a phantom of liver selective internal radiotherapy

The aim of this study is to determine the feasibility of achieving quantitative measurement in (90)Y-microspheres liver selective internal radiotherapy (SIRT) by imaging (90)Y with a conventional non-time of flight (TOF) PET device. Instead of the bremsstrahlung x-rays of the β-decay, the low branch of e(-)- e(+) pair production in the (90)Y-decay was used. The activity distribution in a phantom-simulated liver SIRT was obtained by direct (90)Y-PET imaging. We tested a LYSO TOF PET and two GSO and BGO non-TOF PET scanners using a 3.6-l cylindrical phantom filled with the (90)Y solution containing two sets of hot and cold spheres. The best hot contrast was obtained with the LYSO TOF. It was close to the expected value and remained constant, even for short acquisition times. The LYSO non-TOF was about 10% lower. The GSO performed similarly but degraded for shorter times whilst the BGO was the worst with 40% loss. For the cold spheres, the LYSO TOF and the GSO provided the best results, while the LYSO non-TOF and the BGO were the worst. (90)Y PET imaging in liver SIRT is achievable with LYSO TOF. Conventional LYSO and GSO show a loss of contrast and require longer acquisition times. BGO imaging is not feasible for dosimetry calculation.

Beta shape-factor function and activity determination of 241Pu

The Physikalisch-Technische Bundesanstalt (PTB) investigated the low-energy beta emitter (241)Pu within the scope of an international key comparison on the activity concentration of the same solution. The activity concentration was measured by means of liquid scintillation counters with two and three photomultiplier tubes (PMT). The counting efficiencies were determined with two established techniques, which are based on a free parameter model. The free parameter is determined via (3)H-efficiency tracing in systems with two PMTs, or it is derived from the triple-to-double coincidence ratio (TDCR) in a system with three PMTs. Both methods require an accurate computation of the beta emission spectrum of the first-forbidden (non-unique) transition. In this work, the experimental outcome of a recent measurement from Loidl et al. (2010) with cryogenic magnetic calorimeters was used to determine a shape-factor function. The computed beta spectrum is in good agreement with the measured data when the shape-factor function C(W)=1-1.9582W+0.96078 W(2) and an end-point energy E(β,max)=21.6 keV are used. The activity concentrations determined with the two methods agree well when using the new shape-factor function, whereas a considerable discrepancy is found when assuming C(W)=1, as for an allowed beta transition. Consequently, the difference between the efficiency tracing method and the TDCR method, as observed by other researchers, could be resolved.

Determination of the shape factor of (90)Sr by means of the cutoff energy yield method

Usually, Kurie plots are used to analyze beta-spectra shape-factor functions measured by means of semiconductor and magnetic spectrometers. A drawback of these techniques is the occurrence of self-absorption within the samples through which the emission spectrum is altered. In liquid-scintillation samples self-absorption does not occur, but the poor energy resolution makes the analysis of the spectra difficult. To overcome this problem, two resolution-invariant observables are used for determining the shape-factor function of (90)Sr: (1) the maximum point energy and (2) the cutoff energy yield. The measured shape-factor function of (90)Sr agrees with the one which is predicted by theory for the first-forbidden unique transition.

Activity standardisation of 18F and ionisation chamber calibration for nuclear medicine

Primary activity standardisations were performed on solutions of (18)F using 4pibeta-gamma coincidence counting and liquid scintillation counting (LSC) according to the CIEMAT/NIST method. A beta(+)-emission probability of 96.86% was used for both methods. The various standardised (18)F solutions were measured in ionisation chambers of the Physikalisch-Technische Bundesanstalt (PTB) and compared by determining radionuclide calibration factors. Already in 2001 an (18)F solution had been standardised at the PTB and compared with the results of nine national metrology institutes (NMIs), using the ISOCAL IV secondary radionuclide calibrators of the National Physical Laboratory (NPL) as transfer instruments and a (68)Ge check source solution. These results were linked to the International Reference System (SIR) at the Bureau International des Poids et Mesures (BIPM) by aliquots of solutions sent by the Laboratoire National Henri Becquerel (BNM-LNHB) and the NPL. Further on, in 2005, PTB sent an aliquot of an (18)F solution to the SIR for ionisation chamber measurements. A value of the equivalent activity was determined and included in the key comparison database (KCDB). The recent PTB value of the equivalent activity of the SIR is in good agreement with the key comparison reference value determined from five NMIs. These results confirm that the standardisation of (18)F solutions can be achieved with the accuracy required for use in nuclear medicine and, in particular, for applications in positron emission tomography (PET).

A new internal pair production branching ratio of 90Y: the development of a non-destructive assay for 90Y and 90Sr

(90)Y is utilized as a therapeutic radioisotope in radiolabeled monoclonal antibodies and in microspheres for targeted radiation therapy of the liver. Currently, the widely used dose calibrator assay of (90)Y can have uncertainties exceeding +/-10%. A non-destructive assay using spectroscopy is possible by reducing the currently published uncertainty (+/-12%) in the internal pair production branching ratio for the 0(+)-0(+) transition of (90)Zr. A high-purity germanium detector was used to determine the branching ratio to be (31.86+/-0.47) x 10(-6).

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.

Radiopharmaceutical management of90Y/111In labeled antibodies: shielding and quantification during preparation and administration

BACKGROUND

The combined application of potent beta-emitting isotopes for therapy with remitting isotopes for scintigraphy requires a profound regimen concerning team member safety and radionuclide quantification.

METHODS

We have developed materials and methods for a proper and easy manipulation of 90Y during preparation and administration of 90Y/111In pharmaceuticals used for radioimmunotherapy.

RESULTS

The efficacy of the shielding measures is documented. Protocols for the calibration of gamma-dose calibrators with respect to 90Y are extended to the assessment of quench-corrected liquid scintillation counting of 90Y. The contribution of 90Y backscatter to 111 In counting is quantified. Newly developed shielding equipment allows an adequate administration of relatively large volumes (100 ml) of 90Y/111In labeled pharmaceuticals to patients.

CONCLUSIONS

The procedures described combine pharmaceutical (Good Manufacturing Practice) and radiation safety requirements with an accurate logging of relevant data.

Half-life evaluations for 3H, 90Sr, and 90Y

A recent paper has reviewed methods for the evaluation of discrepant sets of data and demonstrated the results of applying these methods to the published half-life data of 90Sr and 137Cs [MacMahon, T.D., Pearce, A., Harris, P., 2004. Convergence of techniques for the evaluation of discrepant data. Appl. Radiat. Isot. 60, 275-281]. The half-life data for 3H has been subject to a comprehensive review and critical evaluation by Lucas and Unterweger [2000. Comprehensive review and critical evaluation of the half-life of tritium. J. Res. Natl. Inst. Stand. Technol. 105, 541-549]. The current paper reports the results of applying the various evaluation procedures of MacMahon et al. Convergence of techniques for the evaluation of discrepant data. Appl. Radiat. Isot. 60, 275-281] to the data of Lucas and Unterweger [Comprehensive review and critical evaluation of the half-life of tritium. J. Res. Natl. Inst. Stand. Technol. 105, 541-549], resulting in a recommended half-life of 4497(4) days. MacMahon et al. [Convergence of techniques for the evaluation of discrepant data. Appl. Radiat. Isot. 60, 275-281] highlighted problems in the evaluation of the discrepant half-life data of 90Sr, in particular the worrying upward trend in the data, where the weighted mean of all the measurements increases, on average, by 35 days each time a new measurement result is added. The current paper reports on further analyses of these data. New measurements of the half-life of 90Y have been reported by Kossert and Schrader [2004. Standardization by liquid scintillation counting and half-life measurements of 90Y. Appl. Radiat. Isot. 60, 741]. This has prompted a new evaluation of all available published 90Y half-life data. The data are fairly consistent, and a value of 64.063(16) h is recommended.

Development of activity standard for 90Y microspheres

(90)Y microspheres are important therapeutic radiopharmaceuticals used in the treatment of liver cancer through a process known as selective internal radiation therapy. SIR-spheres is a radiopharmaceutical product that is comprised of (90)Y microspheres suspended in sterile, pyrogen-free water for injection into patients. It is necessary to establish for the SIR-spheres production the capability of accurately measuring the activity of this product to a traceable national measurement standard. An activity standard for SIR-spheres was developed from a standard for (90)Y solution, employing a highly quantifiable chemical digestion process. Calibration factors for the manufacturer's ionisation chambers were determined for 1 and 5 ml of the SIR-spheres product placed in Wheaton vials, for both 34% and 44% of (90)Y microsphere concentration.