Targeted internal radiotherapy is a promising new treatment method for metastatic cancer. Several scandium isotopes, Sc-43,44,47, could serve as paired diagnostic and therapeutic (or “theranostic”) isotopes for these diseases. While ideal production routes for diagnostic Sc-43,44 have been found using small medical cyclotrons, research to find a sustainable production method for the therapeutic isotope Sc-47 is ongoing. At the National Superconducting Cyclotron Laboratory (NSCL) and in the future at the Facility for Rare Isotope Beams (FRIB), a supply of Ca-47, the parent of Sc-47, can be produced on a regular basis when the Ca-48 primary beam is used. While the main function of this accelerated primary beam is to produce a user-specified secondary radioactive beam for nuclear physics experiments, >90% of the accelerated primary beam goes unreacted and is collected in beam stops. When a beam blocker with a water interior is used, the Ca-48 primary beam produces Ca-47 as the most abundant fragment resulting from nuclear reactions in the beam blocker. This method of production termed “isotope harvesting”, allows for the use of unreacted, accelerated beams for isotope production. In the future at FRIB, production of Ca-47 through isotope harvesting is predicted to reach the TBq/day level when the Ca-48 is in use. This supply of Ca-47 will facility the production of Ca-47/Sc-47 generators for research quantities of Sc-47. Isotope harvesting methods for the generation of Sc-47, among other interesting products available through this production method, have been explored through several experiments at the NSCL and the Cyclotron Lab at the University of Wisconsin-Madison. An isotope harvesting system has been developed to include a flowing-water target, components to condition and monitor the water, and components to collect radionuclidic products of interest. The durability of this system to irradiation conditions has been tested using a low intensity Ca-40 irradiation and a high intensity proton irradiation. Through Ca-48 irradiations at the NSCL, methods to collect and purify Ca-47 have been tested and optimized. Using these methods, samples of Sc-47 with high radionuclidic purity have been generated and used to radiolabel DTPA-TOC, a biologically active molecule used to target neuroendocrine tumors. This work has verified the feasibility of using isotope harvesting to generate Sc-47 for nuclear medicine applications.
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