  BACKGROUND: 155Tb represents a potentially useful radionuclide for diagnostic
medical applications, but its production remains a challenging problem. A
recent experimental campaign, conducted with low-energy proton beams impinging
on a 155Gd target with 91.9% enrichment, demonstrated a significant
co-production of 156gTb, a contaminant of great concern since its half-life is
comparable to that of 155Tb and its high-energy gamma emissions severely impact
on the dose released and on the quality of the SPECT images. Herein, the
isotopic purity of the enriched 155Gd target necessary to minimize the
co-production of contaminant radioisotopes, in particular 156gTb, was explored
using various computational simulations. RESULTS: Starting from the recent data
obtained with the 155Gd-enriched target, the co-production of other Tb
radioisotopes besides 155Tb has been evaluated using the TALYS code. It was
found that 156Gd, with an isotopic content of 5.87%, was the principal
contributor to the co-production of 156gTb. The analysis also demonstrated that
the maximum amount of 156Gd admissible for 155Tb 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 156Gd 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 155Tb are not severely affected by this
level of impurity in the target. CONCLUSIONS: 155Tb can be produced with a
quality suitable for medical applications using low-energy proton beams and
155Gd-enriched targets if the 156Gd 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 111In, are guaranteed.
