Optimization of Cyclotron Production for Radiometal of Zirconium 89

Zirconium 89 (Zr-89) is a promising radionuclide for development of new PET agents due to its convenient half life of 78.4 h, beta(+) emission rate of 23%, low maximum energy of 0.9 MeV resulting in good spatial resolution, a stable daughter isotope of yttrium-89 (Y-89) and favorable imaging characteristics, with only one significant gamma-line of 909 keV emitted during decay alongside the 511 keV positron photons. Our aim was to share over 2 years of experience of producing isotopically pure Zr-89 via the Y-89(p, n) Zr-89 nuclear reaction with a COSTIS Solid Target System (STS) and CYCLONE 18/9 cyclotron. We optimized the yields without producing either of the long-lived impurities Zr-88 or Y-88. The degradation of the beam energy with 400 and 500 mu m thick niobium foils was tested without overheating problems within 2-6 h of irradiation. From repeated measurements of activity, it was clear that there is a bi-exponential decay of radioactivity due to the short lived Zr-89m and Zr-89. The measured half life of the longer lived radionuclide was consistent with value for Zr-89. The energy spectrum from Zr-89 had energy peaks at 511 keV and 909 keV and was consistent with Zr-89. Production of Zr-89 with 400 (E-p = 9 : 8 MeV) and 500 mu m (E-p = 11.6 MeV) thick niobium beam degrader was achieved, without producing either Zr-88 or Y-88. It was necessary to wait at least 4 hours before measuring the activity and decay correct in order to calculate the Zr-89 activity at the end of cyclotron production. Degrading the proton beam to 10 MeV produces radionuclidically pure Zr-89 with yields from 8 to 9 MBq/mu Ah. Whilst this is enough for pre-clinical use, the yield is not enough for either clinical use or commercial supply. Use of thinner beam degraders (400 mu m) increases the proton beam energy and increases the radionuclidic yield to 15.5 MBq/mu Ah whilst maintaining radionuclidic purity.