Production of the Auger-electron emitter 161Ho for nuclear medicine using proton beams.

Auger-electron emitters have steadily acquired considerable attention in the last decade due to their potential to treat micrometastases. However, the limited availability of Auger-electron emitters has restricted investigations of the radiobiological effects induced by Auger cascades. Among the medically relevant Auger-electron-emitting radiolanthanides, 161Ho stands out as a suitable candidate thanks to its high emission of low-energy electrons. Its low-intensity, low-energy γ rays would enable dosimetry evaluation, while minimizing the deposited dose. This project involved the measurement of proton-induced activation cross sections of holmium radioisotopes, focusing on 160m,161cum,162mHo, using nat,161,162Dy targets irradiated with an 18 MeV cyclotron at the Bern medical cyclotron laboratory. Additionally, production yields were measured at the Paul Scherrer Institute using a 72 MeV separated-sector cyclotron, where dysprosium targets were irradiated for the 161Ho direct production and 165Ho targets to study the indirect production pathway via 161Er decay. Comparison with previously published experimental data showed good agreement for the production cross sections of relevant holmium radioisotopes, while those of 160gHo were reported here for the first time. The nuclear reaction cross sections of 161cum,162g,162mHo were determined by inverting a system of linear equations. Theoretical predictions from the TENDL 2023 library generally overestimated the experimental excitation functions by up to 50%. Irradiation runs of production targets validated the cross sections measured. 165Ho emerged as optimal target material for large-scale production of highly radionuclidically pure 161Ho. Six-hour irradiation of 165Ho targets yielded 15 GBq of 161Er's end-of-bombardment activity at up to 99.98% of radionuclidic purity using 45 MeV proton beams.