155Gd‑target enrichment for terbium‑155 production by low‑energy cyclotrons

Aim: The interest in terbium-155 for SPECT imaging is on the rise thanks to its γ emissions at 87 keV (32%) and 105 keV (25%) and its long half-life that allows to investigate the biodistribution of radiopharmaceuticals over several days. In addition, the possibility to couple it with other Tb radionuclides to produce theranostic pairs increases its appeal for medical purposes [1]. However, an adequate production route for medical applications has not been found yet. In this work the attention is focused on the 155Gd(p,n)155Tb reaction. The co-production of 156Tb has to be limited as much as possible, due to its half-life comparable to the 155Tb one and its high-energy γ emissions that contribute to the dose and compromise the image quality [2]. Materials and methods: The theoretical analysis is crucial to identify the optimal production parameters and irradiation conditions, limiting the co-production of harmful impurities. The theoretical cross sections have been calculated with the TALYS code [3] and compared with the data available in the Literature [4]. Thick-target yields were obtained and dosimetric evaluations were accomplished using the OLINDA software [5], considering an injection of [155Tb]Tb-cm09 [1]. Finally, the dose increase (DI) was determined by combining the yield of all Tb radioisotopes produced with the dosimetric outcomes. Results: With enriched 155Gd targets, the main issue is represented by the presence of 156Gd as impurity. Different levels of 155Gd enrichment have been compared, namely 93.1%, 98%, 99%, and 100%. For each case the assessment of the radionuclidic purity (RNP) and DI have been performed. The dosimetric assessment shows that a maximum 2% content of 156Gd in the target guarantees a safe clinical application. Even though the RNP of reference for the specific case has yet to be established, a 98% RNP value combined with a DI lower than 10% indicates a promising outcome. Conclusion: This work proposes a hospital-cyclotron method for highpurity 155Tb production based on an adequate 155Gd-enrichment of the target. The minimum level of enrichment necessary for its use in clinics has been identified. This is alternative to the use of a post-production mass spectrometry purification proposed in the Literature [4]. Acknowledgements The research is part of the REMIX project, INFN and Legnaro INFN Laboratories.