155 Tb production by cyclotrons: what level of 155 Gd enrichment allows clinical applications?

BACKGROUND: 155 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 155 Gd target with 91.9% enrichment, demonstrated a significant co-production of 156g Tb, a contaminant of great concern since its half-life is comparable to that of 155 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 155 Gd target necessary to minimize the co-production of contaminant radioisotopes, in particular 156g Tb, was explored using various computational simulations.

RESULTS: Starting from the recent experimental data obtained with a 91.9% 155 Gd-enriched target, the co-production of other Tb radioisotopes besides 155 Tb has been theoretically evaluated using the Talys code. It was found that 156 Gd, with an isotopic content of 5.87%, was the principal contributor to the co-production of 156g Tb. The analysis also demonstrated that the maximum amount of 156 Gd admissible for 155 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 156 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 155 Tb are not severely affected by this level of impurity in the target.

CONCLUSIONS: 155 Tb can be produced with a quality suitable for medical applications using low-energy proton beams and 155 Gd-enriched targets, if the 156 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 111 In, are guaranteed.