Development of additive [~(11)C]CO_2 target system in the KOTRON-13 cyclotron and its application for [~(11)C]radiopharmaceutical production

摘要

The KOTRON-13 cyclotron, which was developed in South Korea for the production of medical radioiso-topes, has the structural limitation of only one beam-output port, restricting the production of the car-bon-11 isotope. In the present study, we investigate the design of a switchable target system and develop an effective carbon-11 target in the KOTRON-13 cyclotron, for combination with the fluorine-18 target. The target system was designed by introducing a sliding-type element between the fluorine-18 and carbon-11 targets, a tailor-made C-11 target and its cooling system. For the efficient production of [~(11)C]CO_2, the desirable target shape and internal volume were determined by a Stopping and Range of Ions in Matter (SRIM) simulation program, and the target grid was modified to resist the cavity pressure during beam irradiation. We evaluated the [~(11)C]CO_2 production while varying the material and thickness of the target foil, oxygen content of the nitrogen gas, and target loading pressure. Using sliding-type equipment including an additional gate valve and a high vacuum in a beam line, the bi-directional conversion between the fluorine-18 and carbon-11 targets was efficient regarding the accurate beam irradiation on both targets. The optimal [~(11)C]CO_2 production for 30 min irradiation at 60 μA (86.6 ±1.7 GBq in the target at EOB) was observed at a thickness of 19 μm with HAVAR~® material as a target foil and a target loading pressure of 24 bar with nitrogen plus 300 ppb of oxygen gas. Additionally, the coolant cavity system in the target grid and target chamber is useful to remove the heat transferred to the target body by the internal convection of water and thereby ensure the stability of the [~(11)C]CO_2 production under a high beam current. In the application of C-11 labeled radiopharmaceuticals such as [~(11)C]PIB, [~(11)C]DASB, [~(11)C]PBR28, [~(11)C]Methionine and [~(11)C]Clozapine, the radiochemical yields were shown to be 25-38% (decay corrected) with over 166 GBq/μmol of specific activity. Consequently, the additive carbon-11 target system was successfully developed in only one output port of the KOTRON-13 cyclotron and exhibited the stable production of C-11 labeled radiopharmaceuticals.