Design and evaluation of an external high‐current target for production of ²¹¹At

Abstract

This report presents the design and testing of an external target system for clinical‐scale production of 211At on the Scanditronix MC‐50 cyclotron at the University of Washington Medical Center. While other 211At production sites have typically employed an internal target, an external target setup was deemed necessary at our facility since the cyclotron's main application is for cancer patient treatment via fast neutron therapy. The challenge was that previous external 211At targets demonstrated reduced yields when compared to internal target systems. The target developed in this study has been used in more than 100 irradiations. From those irradiations 211At saturated yields of 181 ± 16 MBq/μA and 241 ± 17 MBq/μA at 28.0 and 29.0 MeV, respectively, were obtained. Importantly, consistent yields were observed over the 25–60 μA range of irradiation currents evaluated. With a recent production in excess of 4 GBq 211At for a 29.0 MeV, 58 μA, 4‐hour irradiation, this external target system has demonstrated its potential for producing clinically relevant quantities of 211At.

Evaluation of a Wet Chemistry Method for Isolation of Cyclotron Produced [211At]Astatine

“…The increase in target size came about when designing and implementing an external target station for increasing our 211 At production levels [29]. It should be noted that our cyclotron situation is somewhat unique, as an external target was required rather than an internal target [43][44][45] because the cyclotron had to remain in daily operation for fast neutron therapy of cancer patients.…”

“…A 0° beam extraction line and external target station were designed and installed to scale-up to clinical production levels of 211 At [29]. Irradiation of a bismuth target (34 mm × 160 mm × 7 mm) placed on a 10° incline with a 50 μA, 29 MeV α-beam routinely produces 19+ mCi of 211 At (as measured in the dose calibrator) in roughly a 45 min irradiation period.…”

“…At the specified depth (0.17 mm), width (18 mm) and length (120 mm) a mass of bismuth of ~3 g can be calculated, but the tolerances for milling the cavity in the aluminum backing can change the mass significantly. The bismuth target was installed on the beamline as previously described [29].…”

“…Even with process optimization, the recovered yields from the dry distillation varied greatly at times. In addition to that problem, more recently our bismuth target was increased in size to produce higher quantities of 211 At required to undertake clinical studies [29]. The increase in the size of bismuth target (and its aluminum backing) resulted in having to greatly increase the size of quartz tube used in the high temperature oven.…”

An automated flow system incorporating in-line acid dissolution of bismuth metal from a cyclotron irradiated target assembly for use in the isolation of astatine-211

“…Astatine-211 is obtained with high radionuclidic purity when the cyclotron beam is maintained at ≤ 29 MeV (Gagnon et al, 2012;Groppi et al, 2005;IAEA, 2009); there are no stable nuclides of this element.…”