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

Purpose: Based on theoretical calculations, the Auger emitter 58m Co 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 58g Co is unfortunately unavoidable, as is ingrowth of the ground state following the isomeric decay of 58m Co. The impact of 58g Co 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 58m Co and 58g Co using a low‐energy cyclotron. Also, the cellular S‐values for 58m Co have been calculated and are presented here for the first time. Methods: 58m Co was produced via the 58 Fe(p,n) 58m Co nuclear reaction on highly enriched 58 Fe metal. In addition, radiochemical separations of produced radio‐cobalt from nat Fe target material were performed. The theoretical subcellular dosimetry calculations for 58m Co and 58g Co 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 of 58m Co was produced in the irradiation corresponding to a production yield of 10.7 MBq/μAh. The activity of 58g Co was measured to be 0.85% ± 0.04% of the produced 58m Co 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 58g Co, the TND and the potency of the 58m Co decrease with time after EOB. If a future treatment with a 58m Co labeled compound is not initiated before, e.g., 21 h after EOB, the resulting TND will be approximately 50% of the TND of ‘pure’ 58m Co as a result of the increased normal tissue dose from the ground state. Conclusions: The Auger emitter 58m Co 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 58g Co requires fast radiochemical processing and use of future 58m Co‐labeled radiopharmaceuticals in order to exploit the high achievable TND of 58m Co.