Tb production by cyclotrons: what level of Gd enrichment allows clinical applications?

Background: Tb represents a potentially useful radionuclide for diagnostic medical applications, but its production remains a challenging problem, in spite of the fact that many production routes have been already investigated and tested. A recent experimental campaign, conducted with low-energy proton beams impinging on a Gd target with 91.9% enrichment, demonstrated a significant co-production of Tb, a contaminant of great concern since its half-life is comparable to that of Tb and its high-energy γ emissions severely impact on the dose released and on the quality of the SPECT images. In the present investigation, the isotopic purity of the enriched Gd target necessary to minimize the co-production of contaminant radioisotopes, in particular Tb, was explored using various computational simulations.

Results: Starting from the recent experimental data obtained with a 91.9% Gd-enriched target, the co-production of other Tb radioisotopes besides Tb has been theoretically evaluated using the Talys code. It was found that Gd, with an isotopic content of 5.87%, was the principal contributor to the co-production of Tb. The analysis also demonstrated that the maximum amount of Gd admissible for Tb production with a radionuclidic purity higher than 99% was 1%. A less stringent condition was obtained through computational dosimetry analysis, suggesting that a 2% content of Gd in the target can be tolerated to limit the dose increase to the patient below the 10% limit. Moreover, it has been demonstrated that the imaging properties of the produced Tb are not severely affected by this level of impurity in the target.

Conclusions: Tb can be produced with a quality suitable for medical applications using low-energy proton beams and Gd-enriched targets, if the Gd impurity content does not exceed 2%. Under these conditions, the dose increase due to the presence of contaminant radioisotopes remains below the 10% limit and good quality images, comparable to those of In, are guaranteed.