Experimental determination of calibration settings of a commercially available radionuclide calibrator for various clinical measurement geometries and radionuclides

Precise activity measurements of medical radionuclides using an ionization chamber: a case study with Terbium-161

Background

¹⁶¹Tb draws an increasing interest in nuclear medicine for therapeutic applications. More than 99% of the emitted gamma and X-rays of ¹⁶¹Tb have an energy below 100 keV. Consequently, precise activity measurement of ¹⁶¹Tb becomes inaccurate with radionuclide dose calibrators when using inappropriate containers or calibration factors to account for the attenuation of this low energy radiation. To evaluate the ionization chamber response, the sample activity must be well known. This can be performed using standards traceable to the Système International de Référence, which is briefly described as well as the method to standardize the radionuclides.

Methods

In this study, the response of an ionization chamber using different container types and volumes was assessed using ¹⁶¹Tb. The containers were filled with a standardized activity solution of ¹⁶¹Tb and measured with a dedicated ionization chamber, providing an accurate response. The results were compared with standardized solutions of high-energy gamma-emitting radionuclides such as ¹³⁷Cs, ⁶⁰Co, ¹³³Ba and ⁵⁷Co.

Results

For the glass vial type with an irregular glass thickness, the ¹⁶¹Tb measurements gave a deviation of 4.5% between two vials of the same type. The other glass vial types have a much more regular thickness and no discrepancy was observed in the response of the ionization chamber for these type of vials. Measurements with a plastic Eppendorf tube showed stable response, with greater sensitivity than the glass vials.

Conclusion

Ionization chamber measurements for low-energy gamma emitters (< 100 keV), show deviation depending on the container type used. Therefore, a careful selection of the container type must be done for activity assessment of ¹⁶¹Tb using radionuclide dose calibrators. In conclusion, it was highlighted that appropriate calibration factors must be used for each container geometry when measuring ¹⁶¹Tb and, more generally, for low-energy gamma emitters.

The role of dosimetry and biological effects in metastatic castration-resistant prostate cancer (mCRPC) patients treated with 223Ra: first in human study

BACKGROUND

223Ra is currently used for treatment of metastatic castration resistant prostate cancer patients (mCRPC) bone metastases with fixed standard activity. Individualized treatments, based on adsorbed dose (AD) in target and non-target tissue, are absolutely needed to optimize efficacy while reducing toxicity of α-emitter targeted therapy. This is a pilot first in human clinical trial aimed to correlate dosimetry, clinical response and biological side effects to personalize 223Ra treatment.

METHODS

Out of 20 mCRPC patients who underwent standard 223Ra treatment and dosimetry, in a subset of 5 patients the AD to target and non-target tissues was correlated with clinical effects and radiation-induced chromosome damages. Before each 223Ra administrations, haematological parameters, PSA and ALP values were evaluated. Additional blood samples were obtained baseline (T0), at 7 days (T7), 30 days (T30) and 180 days (T180) to evaluate chromosome damage. After administration WB planar 223Ra images were obtained at 2-4 and 18-24 h. Treatment response and toxicity were monitored with clinical evaluation, bone scan, 18F-choline-PET/CT, PSA value and ALP while haematological parameters were evaluated weekly after 223Ra injection and 2 months after last cycle.

RESULTS

1. a correlation between AD to target and clinical response was evidenced with threshold of 20 Gy as a cut-off to obtain tumor control; 2. the AD to red marrow was lower than 2 Gy in all the patients with no apparently correlation between dosimetry and clinical toxicity. 3. a high dose dependent increase of the number of dicentrics and micronuclei during the course of 223Ra therapy was observed and a linear correlation has been found between blood AD (BAD) and number of dicentrics.

CONCLUSIONS

This study provides some interesting preliminary evidence to be further investigated: dosimetry may be useful to identify a more appropriate 223Ra administered activity predicting AD to target tissue; a dose dependent complex chromosome damage occurs during 223Ra administration and this injury is more evident in heavily pre-treated patients; dosimetry could be used for radioprotection purpose.

TRIAL REGISTRATION

The pilot study has been approved from the Ethics Committee of Regina Elena National Cancer Institute (N:RS1083/18-2111).

Simulation of the response of an ionization chamber to 214Bi emission. Application to the measurement of 222Rn

PENELOPE simulations of a Vinten ionization chamber (IC) were performed to investigate the influence of the thickness of glass-ampoules used in ²²²Rn standardization. The simulation reveals a non-negligible variation of the energy deposited in the chamber gas region (which may induce a proportional variation of the measured current) when considering the β transition emissions of the daughters of ²²²Rn. This reinforces the idea of using a specialist container (made of metal to preserve the integrity of the container) that would circulate between the metrology laboratories in the context of international comparison exercises using the BIPM international reference system (SIR).

A comparison of four radionuclide dose calibrators using various radionuclides and measurement geometries clinically used in nuclear medicine

PURPOSE

Reliable quantification of radioactivity in nuclear medicine is becoming increasingly important in various therapeutic applications requiring a high accuracy of nuclear medicine measuring equipment, such as radionuclide calibrators. In this study the accuracy of four different radionuclide calibrators was assessed for ^99mTc, ¹¹¹In, ⁶⁸Ga and ¹⁸F for measurement geometries clinically used.

METHODS

Syringes and vials were prepared with a reference activity using a stock solution of which the activity concentration was determined using gamma-ray spectroscopy. The accuracy of four different radionuclide calibrator systems, ISOMED 2000, ISOMED 2010, VIK-202 and Capintec CRC-25R, was assessed by comparing the measured activity to the reference activity.

RESULTS

Deviations in measured activity from reference values were found up to 12.5%, 32.0%, 29.0% and 12.6% for ^99mTc, ¹¹¹In, ⁶⁸Ga and ¹⁸F, respectively. For ⁶⁸Ga all radionuclide calibrators systematically overestimated the activity by 10-20%. For ¹¹¹In, large differences in activity measurements were observed between different source geometries, in particular between syringes and vials. Deviations between radionuclide calibrator systems were found up to 11.8%, 44.4%, 14.4% and 8.7% for ^99mTc, ¹¹¹In, ⁶⁸Ga and ¹⁸F, respectively. When comparing similar syringe types of different brands filled with identical stock solution volume, deviations up to 1.8%, 5.8%, 10.2% and 3.2% were found for ^99mTc, ¹¹¹In, ⁶⁸Ga and ¹⁸F.

CONCLUSION

Substantial deviations in measured activity were found for all radionuclides and radionuclide calibrators, which may result in erroneous activity dosing and image quantification. This underlines the importance of thorough validation of radionuclide calibrators for all measurement geometries and radionuclides clinically used.

Calibration setting numbers for dose calibrators for the PET isotopes (52)Mn, (64)Cu, (76)Br, (86)Y, (89)Zr, (124)I

For PET radionuclides, the radioactivity of a sample can be conveniently measured by a dose calibrator. These devices depend on a "calibration setting number", but many recommended settings from manuals were interpolated based on standard sources of other radionuclide(s). We conducted HPGe gamma-ray spectroscopy, resulting in a reference for determining settings in two types of vessels containing one of several PET radionuclides. Our results reiterate the notion that in-house, experimental calibrations are recommended for different radionuclides and vessels.

Investigation of the response variability of ionization chambers for the standard transfer of SIR-Spheres(®)

The present paper addresses the calibration of well-type ionization chambers (ICs) used at LNE-LNHB as standard transfer instruments to calibrate hospitals in the case of SIR-Spheres(®)(90)Y resin microspheres (Sirtex, Australia). Developed for interventional oncology, this radiopharmaceutical is directly injected in the liver for cancer treatment via a selective internal radiation therapy. The present work was carried out in the framework of the European project "Metrology for molecular radiotherapy" (MetroMRT). As commonly performed in radionuclide metrology for radiopharmaceuticals, the objective is to ensure the metrological traceability of SIR-Spheres(®) to hospitals. Preceding studies were focused on primary measurements of SIR-Spheres(®) based on the TDCR (Triple to Double Coincidence Ratio) method, applied after the dissolution of the (90)Y-labeled resin microspheres. As (90)Y is a high-energy β(-)-emitter, the IC response strongly depends on the transport of electrons in the radioactive solution and surroundings (vial, chamber liners and materials). The variability of the IC-response due to the geometry dependence is investigated by means of measurements and Monte Carlo simulations in the case of a Vinten IC. The aim of the present study was also to propose a reliable uncertainty for ICs calibrations for the standard transfer of SIR-Spheres(®) to hospitals.

Deviation in the predefined calibration factors of the dose calibrators and the associated inaccuracy in the radioactivity measurements of beta-gamma emitters

Aim:

To determine whether the predefined calibration factors of the dose calibrators can provide accurate radioactivity measurements of beta-gamma emitters used in routine therapeutic nuclear medicine procedures.

Materials and Methods:

Two models of dose calibrators were used in the present study for radioactivity measurements of ¹⁵³Sm ethylenediamine-N, N, N’, N’-tetrakis methylene phosphonic acid (EDTMP) and ¹⁷⁷Lu (EDTMP). A known (precalibrated) activity of each of the two beta emitters received by us from our National Supplier for administration to the patients with extensive bony metastases for bone pain palliation, was used for experiments.

Results:

When we used the manufacturers’ provided dial setting of 450 × 10, each of the dose calibrators underestimated the radioactivity of ¹⁷⁷Lu by about 9.0%. Dial settings of 403 × 10 and 408 × 10 for ¹⁷⁷Lu on CRC-15R and CRC-ultra dose calibrators respectively were calculated experimentally using an iterative approach. The radioactivity measurements made at these settings provided an excellent agreement with the specified values. Likewise, a dial setting of 230 for each of the two dose calibrators was calculated for ¹⁵³Sm, which provided a good agreement between the experimentally derived radioactivity values and the certified values. A deviation of ± 5.0% was observed when radioactivity of ¹⁷⁷Lu and ¹⁵³Sm was measured over a wide range (4.0 MBq to 2.1 GBq) for time intervals equivalent to 4.5 half-lives of each of the two radionuclides. A deviation of ± 5% was observed when radioactivity was counted in different dilution volumes and in syringes of varying size.

Conclusion:

These variations could lead to a cumulative error of about 20.0% toward the inaccuracy in the radioactivity measurements of the beta-gamma emitters and thus predefined calibration factors of the dose calibrators may require experimental re-setting of these parameters and periodic checking to provide accurate radioactivity estimates of beta-gamma emitters in a given clinical setting.

Automated dispensing and calibration of diagnostic radiopharmaceuticals

A real-time automatic dose dispenser (RADD) has been designed and fabricated for automatic withdrawal and calibration of diagnostic radiopharmaceuticals such as those labelled with (99m)Tc. This system enhances the accuracy and precision of activity measurements and reduces personal radiation exposure. The structure, function, user-friendliness and performance of this device are described and examined for diagnostic activities of (99m)Tc ranging from 50 to 650 MBq. The results show that the RADD minimises the likelihood of miscalibration of radiopharmaceuticals due to human error and results in significantly reduced variability (i.e. higher precision) in dispensed activities of radiopharmaceuticals.

Fatores de correção volumétrica na medição de atividade de 99mTc e 123I em calibradores de radionuclídeos

OBJETIVO: Determinar os fatores de correção para a variação volumétrica dos radiofármacos contidos em recipientes de diferentes geometrias. Comparar a influência desses fatores na determinação da atividade de 99mTc e 123I utilizando dois tipos de calibradores: um com câmara de ionização e outro com detector Geiger-Müller (G-M). Avaliar o desempenho de calibradores de alguns serviços de medicina nuclear em medição de atividade de 99mTc e 123I. MATERIAIS E MÉTODOS: Foram utilizados oito calibradores, frascos de vidro 10R, seringas de plástico de 3 e 5 mL e soluções de 99mTc e 123I. Os fatores de correção foram determinados a partir das medições práticas da variação da leitura do calibrador com a variação do volume da solução no recipiente. O desempenho foi avaliado em relação ao critério de aceitação de ±10% de exatidão exigida pela norma brasileira. RESULTADOS: A variação da resposta do calibrador com a variação do volume do frasco foi bem maior no calibrador que utiliza G-M. Ela também foi maior para 123I do que para 99mTc. CONCLUSÃO: Os resultados confirmam que a resposta dos calibradores depende do volume contido nos recipientes. Essa dependência é mais crítica para os calibradores equipados com detector G-M e para 123I quando comparado com 99mTc.

Comparison of experimental and calculated calibration coefficients for a high sensitivity ionization chamber

The response of a Vacutec 70129 ionization chamber was calculated using the PENELOPE-2008 Monte Carlo code and compared to experimental data. The filling gas mixture composition and its pressure have been determined using IC simulated response adjustment to experimental results. The Monte Carlo simulation revealed a physical effect in the detector response to photons due to the presence of xenon in the chamber. A very good agreement is found between calculated and experimental calibration coefficients for 17 radionuclides.

Usefulness of specific calibration coefficients for gamma-emitting sources measured by radionuclide calibrators in nuclear medicine

PURPOSE

In nuclear medicine, the activity of a radionuclide is measured with a radionuclide calibrator that often has a calibration coefficient independent of the container type and filling.

METHODS

To determine the effect of the container on the accuracy of measuring the activity injected into a patient, The authors simulated a commercial radionuclide calibrator and 18 container types most typically used in clinical practice. The instrument sensitivity was computed for various container thicknesses and filling levels. Monoenergetic photons and electrons as well as seven common radionuclides were considered.

RESULTS

The quality of the simulation with gamma-emitting sources was validated by an agreement with measurements better than 4% in five selected radionuclides. The results show that the measured activity can vary by more than a factor of 2 depending on the type of container. The filling level and the thickness of the container wall only have a marginal effect for radionuclides of high energy but could induce differences up to 4%.

CONCLUSIONS

The authors conclude that radionuclide calibrators should be tailored to the uncertainty required by clinical applications. For most clinical cases, and at least for the low-energy gamma and x-ray emitters, measurements should be performed with calibration coefficients specific to the container type.

Comparison of methods to determine accurate dose calibrator activity measurements

Background

In nuclear medicine, liquid radiopharmaceuticals for diagnostic or therapeutic purposes are administered to patients by using various types of syringes with different volumes. The activity of each "dose" must be carefully measured and documented prior to administration using an activity calibrator.

Methods

Calibrator response is a function of the measurement geometry and, in particular, it depends on the syringe type and filling volume. To minimize the uncertainty associated with the measured activity of the syringe, it is necessary to calculate a calibration curve depending on filling volume for each syringe type. This curve can be obtained by fitting experimentally determined volume correction factors.

Results

A theoretical evaluation of volume correction factors for syringes is reported for three different experimental methods. The aim is to determine the most accurate experimental method among those considered, by examining the expression of uncertainty for the correction factor. This theoretical analysis was then tested experimentally.

Conclusion

The agreement between the experimental data obtained in the constant activity method and gravimetric method at constant specific activity and the small associated uncertainties show the accuracy of these two procedures; while the volumetric method at constant specific activity could lead to a wrong evaluation of the correction factors.