Assessment of radionuclidic impurities in cyclotron produced 99mTc

Introduction: The commercial viability of cyclotron-produced Tc-99m as an alternative to generator-produced Tc-99m depends on several factors. These include: production yield, ease of target processing and recycling of Mo-100, radiochemical purity, specific activity as well as the presence of other radionuclides, particularly various Tc radioisotopes that cannot be separated chemically and will remain in the final clinical preparation. These Tc radionuclidic impurities are derived from nuclear interactions of the accelerated protons with other stable Mo isotopes present in the enriched Mo-100 target. The aim of our study was to determine experimentally the yields of Tc radioisotopes produced from these stable Mo isotopes as a function of incident beam energy in order to predict radionuclidic purity of Tc-99m produced in highly enriched Mo-100 targets of known isotopic composition. Methods: Enriched molybdenum targets of Mo-95, Mo-96, Mo-97, Mo-98 and Mo-100 were prepared by pressing powdered metal into an aluminum target support. The thick targets were bombarded with 10 to 24MeV protons using the external beam line of the U-120 M cyclotron of the Nuclear Physics Institute, Rez. The thick target yields of Tc-94, Tc-94m, Tc-95, Tc-95m, Tc96m+g and Tc-97m were derived from their activities measured by gamma spectrometry using a high purity Ge detector. These data were then used to assess the effect of isotopic composition of highly enriched Mo-100 targets on the radionuclidic purity of Tc-99m as a function of proton beam energy. Estimates were validated by comparison to measured activities of Tc radioisotopes in proton irradiated, highly enriched Mo-100 targets of known isotopic composition. Results: The measured thick target yields of Tc-94, Tc-94m, Tc-95, Tc-95m, Tc96m+g and Tc-97m correspond well with recently published values calculated via the EMPIRE-3 code. However, the measured yields are more favourable with regard to achievable radionuclidic purity of Tc-99m. Reliability of the measured thick target yields was demonstrated by comparison of the estimated and measured activities of Tc-94, Tc-95, Tc-95m, and Tc96m+g in highly enriched Mo-100 (99%) targets that showed good agreement, with maximum differences within estimated uncertainties. Radioisotopes Tc-94m and Tc-97m were not detected in the irradiated Mo-100 targets due to their low activities and measurement conditions: on the other hand we detected small amounts of the short-lived positron emitter Tc-93 (T-1/2=2.75 h). In addition to Tc-99m and trace amounts of the various Tc isotopes, significant activities of Nb-96, Nb-97 and Mo-99 were detected in the irradiated Mo-100 targets. Conclusions: Radioisotope formation during the proton irradiation of Mo targets prepared from different, enriched stable Mo isotopes provides a useful data base to predict the presence of Tc radionuclidic impurities in Tc-99m derived from proton irradiated Mo-100 targets of known isotopic composition. The longer-lived Tc isotopes including Tc-94 (T-1/2=4.883 h), Tc-95 (T-1/2=20.0 h), Tc-95m (T-1/2=61 d), Tc96m+g (T-1/2=4.24 d) and Tc-97m (T-1/2=90 d) are of particular concern since they may affect the dosimetry in clinical applications. Our data demonstrate that cyclotron production of Tc-99m, using highly enriched Mo-100 targets and 19-24 MeV incident proton energy, will result in a product of acceptable radionuclidic purity for applications in nuclear medicine. (C) 2012 Elsevier Inc. All rights reserved.