Production and dosimetric aspects of the potent Auger emitter 58mCo for targeted radionuclide therapy of small tumors

Abstract

Purpose:Based on theoretical calculations, the Auger emitter58mCo has been identified as a potent nuclide for targeted radionuclide therapy of small tumors. During the production of this isotope, the coproduction of the long‐lived ground state 58gCo is unfortunately unavoidable, as is ingrowth of the ground state following the isomeric decay of 58mCo. The impact of 58gCo as a β+‐ and γ‐emitting impurity should be included in the dosimetric analysis. The purpose of this study was to investigate this critical part of dosimetry based on experimentally determined production yields of 58mCo and 58gCo using a low‐energy cyclotron. Also, the cellular S‐values for 58mCo have been calculated and are presented here for the first time.Methods:58mCo was produced via the 58Fe(p,n)58mCo nuclear reaction on highly enriched 58Fe metal. In addition, radiochemical separations of produced radio‐cobalt from natFe target material were performed. The theoretical subcellular dosimetry calculations for 58mCo and 58gCo were performed using the MIRD formalism, and the impact of the increasing ground state impurity on the tumor‐to‐normal‐tissue dose ratios (TND) per disintegration as a function of time after end of bombardment (EOB) was calculated.Results:192 ± 8 MBq of58mCo was produced in the irradiation corresponding to a production yield of 10.7 MBq/μAh. The activity of 58gCo was measured to be 0.85% ± 0.04% of the produced 58mCo activity at EOB. The radio‐cobalt yields in the rapid separations were measured to be >97% with no detectable iron contaminations in the cobalt fractions. Due to the unavoidable coproduction and ingrowth of the long‐lived ground state 58gCo, the TND and the potency of the 58mCo decrease with time after EOB. If a future treatment with a 58mCo labeled compound is not initiated before, e.g., 21 h after EOB, the resulting TND will be approximately 50% of the TND of ‘pure’ 58mCo as a result of the increased normal tissue dose from the ground state.Conclusions: The Auger emitter 58mCo is a potent radioisotope for targeted radionuclide therapy, and the production of therapeutic quantities should be achievable using a small biomedical cyclotron. However, the unavoidable coproduction and ingrowth of the long‐lived ground state 58gCo requires fast radiochemical processing and use of future 58mCo‐labeled radiopharmaceuticals in order to exploit the high achievable TND of 58mCo.