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

Abstract

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.

161Tb-DOTATOC Production Using a Fully Automated Disposable Cassette System: A First Step Toward the Introduction of161Tb into the Clinic

“…Residual radioactivity was detected in the activity vial, the reaction vial, the C18 cartridge, and the waste (Supplemental Table 4). Unlike the bulk product and initial activity measurements, which were performed with appropriate calibration for each container ( 28 ), the residual activity measurements were not comparable to one another, or to the initial activity, because of the distinct geometries of the samples. Nevertheless, the measurements indicated losses mainly in the waste or because of bound activity on the C18 cartridge.…”

“…To ensure sterility, the final product was finally passed through a 0.22-μm filter (Medical Millex-GV Syringe Filter, Hydrophilic Polyvinylidene Fluoride, γ-sterilized; Merck Millipore). At the end of the synthesis, the final product activity and the residual radioactivity remaining in the principal components (activity vial, reaction vial, C18 cartridge, and waste) were measured (Isomed Dose Calibrator 2010) ( 28 ). Aliquots were withdrawn from the total volume for QC purposes.…”

The evaluation of an on-site monitoring program for activity meter quality assurance with exemption-level sources

“…The ideal source geometry should be the same as that of the source being assayed (Carey et al 2012). The ENM type (penicillin vial) was a source geometry for radiochemistry (Juget et al 2022). However, the EFS (point source) was basically used for spectrometry.…”

Comparison of the tolerability of161Tb- and177Lu-labeled somatostatin analogues in the preclinical setting

“…Terbium-161 (in-house produced as previously reported [22] or obtained from Terthera, the Netherlands, via Solumedics AG, Switzerland) and lutetium-177 (ITM Medical Isotopes GmbH, Germany) were measured using calibrated instrumentation (Supplementary Material) [23]. DOTA- LM3 (CSBio, Silicon Valley Menlo Park, California, United States) and DOTATATE (Bachem, Bubendorf, Switzerland) were labeled with terbium-161 or lutetium-177 at a molar activity up to 100 MBq/nmol using standard labeling conditions as previously reported (Supplementary Material) [19].…”