Determination of photon emission probabilities for the main gamma-rays of ²²³Ra in equilibrium with its progeny

Activity standard and calibrations for 227Th with ingrowing progeny

Thorium-227 was separated from its progeny and standardized for activity by the triple-to-double coincidence ratio (TDCR) method of liquid scintillation counting. Confirmatory liquid scintillation-based measurements were made using efficiency tracing with 3H and live-timed anticoincidence counting (LTAC). The separation time and the efficiency of the separation were confirmed by gamma-ray spectrometry. Calibrations for reentrant pressurized ionization chambers, including commercial radionuclide calibrators, and a well-type NaI(Tl) detector are discussed.

Status of the decay data for medical radionuclides: existing and potential diagnostic γ emitters, diagnostic β+ emitters and therapeutic radioisotopes

Recommended half-lives and specific well-defined emission energies and absolute emission probabilities are important input parameters that should be well-defined to assist in ensuring the diagnostic and therapeutic efficacy of individual radionuclides when applied in the field of nuclear medicine. Bearing in mind the nature of these requirements, approximately one hundred radionuclides have been considered and re-assessed as to whether their decay data are either adequately quantified, or require further in-depth measurements to improve their existing status and merit full re-evaluations of their decay schemes. The primary aim of such a review is to provide sufficient information on the existing and future requirements for such atomic and nuclear data.

Radium-223 Dichloride in Peritoneal Dialysate Following Treatment of Metastatic Castration-resistant Prostate Cancer

ABSTRACT

Radium-223 dichloride was administrated to a patient with castration-resistant metastatic prostate cancer (mCRPC) undergoing in-home peritoneal dialysis. The peritoneal dialysate and tubing sets were collected following four treatment cycles and analyzed for removable contamination, dose rate, and radioactivity. The purpose of the study was to assess radiation safety concerns and compare data with patients having normal renal function. Sixty-two liters of dialysate in 22 samples were collected over 4 mo. The mean surface dose rate of the dialysate bags was 0.16 μSv h-1 (range 0.11-0.23 μSv h-1). The highest measured removable surface activity was 150 dpm per 100 cm2. The mean cumulative percent of administered activity was 1.7% (range 1.4-1.9%). The mean concentration of 223Ra in dialysate was 4.0 kBq L-1 (range 0.56-14.1 kBq L-1). There was no association between the measured 223Ra activity in dialysate with peritoneal dwell time or the number of exchanges following administration. The measurement of the 211Bi and 223Ra ratio in the dialysate compared with a standard showed an increase of 23% by 40 h post administration. The data presented suggest that 223Ra dialysate can be safely managed in the home without risk of radiation dose or contamination if minimal precautions are taken. Patients with normal renal function have been shown to excrete up to six times more 223Ra in urine compared to those undergoing hemodialysis or peritoneal dialysis. A potential consequence may be an increase in 223Ra activity transiting GI tract leading to intestinal effects.

Activity standardisation of 223Ra

We report here on the primary activity standardisation of a²²³Ra dichloride solution in equilibrium with its decay daughters. Both the triple-to-double-coincidence-ratio (TDCR) method with an in-house TDCR detector and the CIEMAT-NIST efficiency tracing (CNET) technique with a commercial counter were used. The liquid scintillation efficiencies for both methods are about 6 while the activities they predict with about 0.4% relative standard uncertainty agree within 0.15%. For backup, the solution was also standardised with 4πγ NaI(Tl) integral counting with a well-type NaI(Tl) detector, and efficiencies computed by Monte Carlo simulations using the GEANT code. This simple technique, unused previously for this nuclide, yielded an activity concentration compatible with, but 1% lower than, the one determined by liquid scintillation counting.

Ra-224 activity, half-life, and 241 keV gamma ray absolute emission intensity: A NIST-NPL bilateral comparison

The national metrology institutes for the United Kingdom (UK) and the United States of America (USA) have compared activity standards for 224Ra, an α-particle emitter of interest as the basis for therapeutic radiopharmaceuticals. Solutions of 224RaCl2 were assayed by absolute methods, including digital coincidence counting and triple-to-double coincidence ratio liquid scintillation counting. Ionization chamber and high-purity germanium (HPGe) γ-ray spectrometry calibrations were compared; further, a solution was shipped between laboratories for a direct comparison by HPGe spectrometry. New determinations of the absolute emission intensity for the 241 keV γ ray (Iγ = 4.011(16) per 100 disintegrations of 224Ra) and of the 224Ra half-life (T1/2 = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.

Feasibility and limitations of quantitative SPECT for 223Ra

The aim of this paper is to investigate the feasibility and limitations of activity-concentration estimation for ²²³Ra using SPECT. Phantom measurements are performed using spheres (volumes 5.5 mL to 26.4 mL, concentrations 1.6 kBq mL^-1 to 4.5 kBq mL^-1). Furthermore, SPECT projections are simulated using the SIMIND Monte Carlo program for two geometries, one similar to the physical phantom and the other being an anthropomorphic phantom with added lesions (volumes 34 mL to 100 mL, concentrations 0.5 kBq mL^-1 to 4 kBq mL^-1). Medium-energy and high-energy collimators, 60 projections with 55 s per projection and a 20% energy window at 82 keV are employed. For the Monte Carlo simulated images, Poisson-distributed noise is added in ten noise realizations. Reconstruction is performed (OS-EM, 40 iterations, 6 subsets) employing compensation for attenuation, scatter, and collimator-detector response. The estimated concentrations in the anthropomorphic phantom are also corrected using recovery coefficients. Errors for the largest sphere in the physical phantom range from -25% to -34% for the medium-energy collimator and larger deviations for smaller spheres. Corresponding results for the high-energy collimator are -15% to -31%. The corresponding Monte Carlo simulations show standard deviations of a few percentage points. For the anthropomorphic phantom, before application of recovery coefficients the bias ranges from -16% to -46% (medium-energy collimator) and -10% to -28% (high-energy collimator), with standard deviations of 2% to 14% and 1% to 16%. After the application of recovery coefficients, the biases range from -3% to -35% (medium energy collimator) and from 0% to -18%. The errors decrease with increasing concentrations. Activity-concentration estimation of ²²³Ra with SPECT is feasible, but problems with repeatability need to be further studied.

α-particle discrimination in the measurement of α/β decaying chains by the use of ultra-thin plastic scintillation sheets

We found that problems of α/β pulse-height overlapping can be solved by use of ultra-thin plastic scintillator sheets. In order to determine individual α-counting efficiencies for nuclides in the ²²³Ra decay-chain, the 4πα-γ anti-coincidence spectrometry technique was employed with a 4πα-γ detector configuration composed of a source sandwiched between ultra-thin plastic scintillator sheets and a Ge detector. This technique was applied to the activity measurements of ²²³Ra. This type of sandwich source is useful to prevent possible leakage of ²¹⁹Rn gas.

A Study of 219Rn Outgassing and 211Pb Contamination from 223Ra In dry, Liquid, and Murine Tissue Samples

INTRODUCTION

A study of Pb contamination caused by the outgassing of Rn from Ra in dry, liquid, and murine tissues samples has been made to help design proper handling procedures for Ra in preclinical biodistribution work.

MATERIALS AND METHODS

Pb activity levels were measured from Ra in dry, liquid, and tissue samples using aspiration and autoradiography techniques.

RESULTS

Using aspiration techniques on dry samples of Ra, an average Rn outgassing rate of 51% ± 21% was measured with one measurement reaching as high as 81%. 31% ± 4% Pb contamination was measured within a 4.3 cm radius of a dry Ra source placed inside a 10-cm-diameter petri dish where the lip of the petri dish contained the Rn dissemination. Without the containment of the petri dish, Rn can reach as far as 7.8 cm from the source with trace levels spreading further. Using aspiration techniques on liquid samples of Ra, outgassing rates of Rn were 0.9% ± 0.3%. The outgassing levels in harvested organs from a biodistribution were as high as 10.1% ± 0.4% for an intraperitoneally injected mouse and 0.204% ± 0.006% for an intravenously injected mouse. The outgassing of the intravenously injected mouse carcass was less than 0.1%.

CONCLUSION

In dry form, the high levels of Rn outgassing from a Ra source necessitate the use of ventilated biohoods when handling or preparing dry Ra from source vials. The very low levels of Rn outgassing from Ra liquid sources reduces exposure to Rn by a factor of 50. Rn exposure from murine organ tissue reaches levels of 10% when handling organs from an intraperitoneal injection and less than 0.2% for an intravenous injection.

A new evaluation of the nuclear decay data of 223Ra

Since 2013, the radionuclide 223Ra is used in nuclear medicine to prepare radiopharmaceuticals for targeted radiotherapy. 223Ra is a member of the natural radioactive series of actinium and decays by alpha-particle emission, populating the excited levels of 219Rn. According to the 2011 nuclear decay data evaluation within the Decay Data Evaluation Project (DDEP), by V.P. Chechev, the half-life of 223Ra is (11.43 ± 0.03) days. The decay scheme is not considered as fully complete, because of the disagreement between the measured and calculated probabilities of some alpha-transitions and incomplete information on several gamma-ray transitions. New high quality measurements of 223Ra nuclear decay data were performed and numerous results were published since 2012 and, consequently, an updated nuclear decay data evaluation was undertaken, according to the DDEP procedures. The main results obtained, focusing on the recommended data improvements, are presented and discussed in this paper.

The potential radio-immunotherapeutic α-emitter 227Th - part I: Standardisation via primary liquid scintillation techniques and decay progeny ingrowth measurements

Thorium-227 is a potential therapeutic radionuclide for applications in targeted α-radioimmunotherapy for the treatment of various types of cancer. To provide nuclear medicine departments involved in Phase I clinical trials traceability to the SI unit of radioactivity (Bq), a standardisation of a radiochemically pure ²²⁷Th aqueous solution has been performed at the National Physical Laboratory. This was achieved via two primary liquid scintillation (LS) techniques -4π(LS)-γ digital coincidence counting (DCC) and 4π LS counting. These absolute techniques were supported by the indirect determination of the ²²⁷Th activity via the measurement of the ingrowth and decay rate of the decay progeny by both ionisations chambers and high purity germanium (HPGe) gamma-ray spectrometry. The results of the primary techniques were found to be consistent, both with each other (zeta score = 1.1) and to the decay progeny ingrowth measurements. An activity per unit mass of 20.726 (51) kBq g^-1 was determined for the solution. A procedure has been developed that provided an effective separation of the ²²⁷Th from its decay progeny, which was shown by the effective time zero of the ²²⁷Th-²²³Ra nuclear chronometer measured by HPGe gamma-ray spectrometry.

Measurement of absolute γ-ray emission probabilities in the decay of 227Ac in equilibrium with its progeny

The emission probabilities of γ rays produced in the ²²⁷Ac decay series were determined by high-resolution γ-ray spectrometry of sources with standardised activity. The sources were prepared quantitatively on glass discs by drop deposition of a solution with ²²⁷Ac in radioactive equilibrium with its daughter nuclides. Their activity was measured by a primary standardisation technique based on alpha-particle counting at a defined low solid angle. Four laboratories performed γ-ray spectrometry and derived absolute γ-ray intensities. Mean values were calculated and compared with literature data and the currently recommended evaluated data. New values on certain γ-ray emission probabilities are proposed.

RECOMMENDATIONS FOR MONITORING AND INTERNAL DOSIMETRY FOR NUCLEAR MEDICINE STAFF EXPOSED TO RADIOPHARMACEUTICALS 223Ra DICHLORIDE

223Ra is a radiopharmaceutical used as unsealed source in nuclear medicine. In the case of staff inhalation contamination of 223Ra, methods to estimate the committed effective dose should be chosen with care. Three methods are available: whole-body measurement and gamma spectrometry for urine or faeces samples. Considering the analytical performances and uncertainties of these three methods, we propose recommendations for special dose assessment. As a first choice, due to its rapidity and its non-invasiveness, an in vivo analysis (with HPGe detector) is the most appropriate method. However, after 24 h, whole-body counting is not sensitive enough to detect a minimum effective dose of 1 mSv. Sufficient sensitivity can only be reached up to 8 days after contamination by true 24 h faeces samples analyses. Thus, despite its main drawbacks, this method appears to be more appropriate than urine to estimate the committed effective dose in addition to whole-body counting.

Assessment of collimators in radium-223 imaging with channelized Hotelling observer: a simulation study

OBJECTIVE

Radium-223 (²²³Ra) is used in unsealed radionuclide therapy for metastatic bone tumors. The aim of this study is to apply a computational model observer to ²²³Ra planar images, and to assess the performance of collimators in ²²³Ra imaging.

METHODS

The ²²³Ra planar images were created via an in-house Monte Carlo simulation code using HEXAGON and NAI modules. The phantom was a National Electrical Manufacturers Association body phantom with a hot sphere. The concentration of the background was 55 Bq/mL, and the sphere was approximately 1.5-20 times that of the background concentration. The acquisition time was 10 min. The photopeaks (and the energy window) were 84 (full width of energy window: 20%), 154 (15%), and 270 keV (10%). Each 40 images, with and without hot concentration, were applied to a three-channel difference-of-Gaussian channelized Hotelling observer (CHO), and the signal-to-noise ratio (SNR) of the hot region was calculated. The images were examined using five different collimators: two low-energy general-purpose (LEGP), two medium-energy general-purpose (MEGP), and one high-energy general-purpose (HEGP) collimators.

RESULTS

The SNR value was linearly proportional to the contrast of the hot region for all collimators and energy windows. The images of the 84-keV energy window with the MEGP collimator that have thicker septa and larger holes produced the highest SNR value. The SNR values of two LEGP collimators were approximately half of the MEGP collimators. The HEGP collimator was halfway between the MEGP and LEGP. Similar characteristics were observed for other energy windows (154, 270 keV). The SNR value of images captured via the 270-keV energy window was larger than 154-keV, although the sensitivity of the 270-keV energy window is lower than 154-keV. The results suggested a positive correlation between the SNR value and the fraction of unscattered photons.

CONCLUSIONS

The SNR value of CHO reflected the performance of collimators and was available to assess and quantitatively evaluate the collimator performance in ²²³Ra imaging. The SNR value depends on the magnitudes of unscattered photon count and the fraction of unscattered photon count. Consequently, in this study, MEGP collimators performed better than LEGP and HEGP collimators for ²²³Ra imaging.

Ra-223 SPECT for semi-quantitative analysis in comparison with Tc-99m HMDP SPECT: phantom study and initial clinical experience

BACKGROUND

Image-based measurement of absorbed dose of Ra-223 dichloride may be useful in predicting therapeutic outcome in patients with castration-resistant prostate cancer (CRPC). In general, SPECT has been found to be more accurate than planar imaging in terms of lesion-based analysis. The aims of this study were to assess the feasibility and clinical usefulness of Ra-223 SPECT. The energy spectrum of Ra-223 and SPECT images of a cylindrical phantom with a hot rod were obtained to determine the collimator candidates and energy window settings suitable for clinical Ra-223 SPECT (basic study A). Another phantom with a tube-shaped chamber and two spheres simulating bowel activity and metastatic lesions in the lumbar spine was scanned with medium-energy general-purpose (MEGP) and high-energy general-purpose (HEGP) collimators (basic study B). Ten patients with CRPC underwent SPECT imaging 2 h after Ra-223 injection successively with MEGP and HEGP collimators in random order for 30 min each. Lesion detectability and semi-quantitative analyses of bone metastasis (i.e. lesion-to-background ratio (LBR)) were performed compared to Tc-99m HMDP SPECT.

RESULTS

Basic study A revealed that an 84-keV photopeak ± 20% using the HEGP collimator offers better SPECT image quality than the other imaging conditions. Basic study B showed that uptake in one of the spheres was overestimated by overlapped activity of the tube-shaped chamber in planar imaging whereas the spheres had similar counts and significantly higher sphere-to-background ratio in SPECT. On both planar and SPECT images, HEGP gave higher image contrast than MEGP (p < 0.01). In the clinical study, Ra-223 SPECT at 84 keV ± 20% depicted more lesions with the HEGP than with the MEGP collimator (51 vs 36, p = 0.013). There was a positive correlation between LBR in Tc-99m SPECT and in Ra-223 SPECT (r = 0.67 with the MEGP and 0.69 with the HEGP collimator, p < 0.01). LBRs were significantly higher with the HEGP than with the MEGP collimator (p < 0.01).

CONCLUSIONS

We recommended the use of the HEGP collimator at 84 keV ± 20% for Ra-223 SPECT imaging. Lesion-based semi-quantitative analysis in the human study revealed a good correlation between Ra-223 and Tc-99m HMDP SPECT in the early phase (2-3 h post injection).

A Monte Carlo study on (223)Ra imaging for unsealed radionuclide therapy

PURPOSE

Radium-223 ((223)Ra), an α-emitting radionuclide, is used in unsealed radionuclide therapy for metastatic bone tumors. The demand for qualitative (223)Ra imaging is growing to optimize dosimetry. The authors simulated (223)Ra imaging using an in-house Monte Carlo simulation code and investigated the feasibility and utility of (223)Ra imaging.

METHODS

The Monte Carlo code comprises two modules, hexagon and nai. The hexagon code simulates the photon and electron interactions in the tissues and collimator, and the nai code simulates the response of the NaI detector system. A 3D numeric phantom created using computed tomography images of a chest phantom was installed in the hexagon code. (223)Ra accumulated in a part of the spine, and three x-rays and 19 γ rays between 80 and 450 keV were selected as the emitted photons. To evaluate the quality of the (223)Ra imaging, the authors also simulated technetium-99m ((99m)Tc) imaging under the same conditions and compared the results.

RESULTS

The sensitivities of the three photopeaks were 147 counts per unit of source activity (cps MBq(-1); photopeak: 84 keV, full width of energy window: 20%), 166 cps MBq(-1) (154 keV, 15%), and 158 cps MBq(-1) (270 keV, 10%) for a low-energy general-purpose (LEGP) collimator, and those for the medium-energy general-purpose (MEGP) collimator were 33, 13, and 8.0 cps MBq(-1), respectively. In the case of (99m)Tc, the sensitivity was 55 cps MBq(-1) (141 keV, 20%) for LEGP and 52 cps MBq(-1) for MEGP. The fractions of unscattered photons of the total photons reflecting the image quality were 0.09 (84 keV), 0.03 (154 keV), and 0.02 (270 keV) for the LEGP collimator and 0.41, 0.25, and 0.50 for the MEGP collimator, respectively. Conversely, this fraction was approximately 0.65 for the simulated (99m)Tc imaging. The sensitivity with the LEGP collimator appeared very high. However, almost all of the counts were because of photons that penetrated or were scattered in the collimator; therefore, the proportions of unscattered photons were small.

CONCLUSIONS

Their simulation study revealed that the most promising scheme for (223)Ra imaging is an 84-keV window using an MEGP collimator. The sensitivity of the photopeaks above 100 keV is too low for (223)Ra imaging. A comparison of the fractions of unscattered photons reveals that the sensitivity and image quality are approximately two-thirds of those for (99m)Tc imaging.