Abstract
Thulium-167 is a promising radionuclide for nuclear medicine applications with potential use for both diagnosis and therapy (“theragnostics”) in disseminated tumor cells and small metastases, due to suitable gamma-line as well as conversion/Auger electron energies. However, adequate delivery methods are yet to be developed and accompanying radiobiological effects to be investigated, demanding the availability of 167Tm in appropriate activities and quality. We report herein on the production of radionuclidically pure 167Tm from proton-irradiated natural erbium oxide targets at a cyclotron and subsequent ion beam mass separation at the CERN-MEDICIS facility, with a particular focus on the process efficiency. Development of the mass separation process with studies on stable 169Tm yielded 65 and 60% for pure and erbium-excess samples. An enhancement factor of thulium ion beam over that of erbium of up to several 104 was shown by utilizing laser resonance ionization and exploiting differences in their vapor pressures. Three 167Tm samples produced at the IP2 irradiation station, receiving 22.8 MeV protons from Injector II at Paul Scherrer Institute (PSI), were mass separated with collected radionuclide efficiencies between 11 and 20%. Ion beam sputtering from the collection foils was identified as a limiting factor. In-situ gamma-measurements showed that up to 45% separation efficiency could be fully collected if these limits are overcome. Comparative analyses show possible neighboring mass suppression factors of more than 1,000, and overall 167Tm/Er purity increase in the same range. Both the actual achieved collection and separation efficiencies present the highest values for the mass separation of external radionuclide sources at MEDICIS to date.
Highlights
Auger electrons (AEs) can be highly radiotoxic when they decay in the vicinity of DNA in the cell nucleus [1, 2], which makes them attractive for radiotherapy
Taking the extracted 167Tm as performance of the process without implantation into account, these results prove that an extraction and implantation point delivery efficiency of 167Tm from an irradiated natEr2O3 target between 27 and 45% is possible at MEDICIS if sputtering can be avoided
This work provides a detailed description on the steps followed to establish the mass separation process of external 167Tm samples, produced by proton irradiation of erbium oxide at Paul Scherrer Institute (PSI), at the CERN-MEDICIS facility
Summary
Auger electrons (AEs) can be highly radiotoxic when they decay in the vicinity of DNA in the cell nucleus [1, 2], which makes them attractive for radiotherapy. To date, clinical research has been very limited and there is still much to learn about the molecular and cellular radiobiological effects of AEs. Most of the AE-emitting radionuclides emit conversion electrons with higher energies compared to AEs, resulting in a longer-range effect up to several micrometers [4, 5] and less dense ionization. Pt isotopes were shown to be highly radiotoxic due to the high emission rates (due to the excess of 30 Auger electrons per decay on average) [5]. Low specific activity and no satisfactory radiolabeling strategies obstruct their further investigation
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