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

Purpose: Based on theoretical calculations, the Auger emitter Co-58m 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 Co-58g is unfortunately unavoidable, as is ingrowth of the ground state following the isomeric decay of Co-58m. The impact of Co-58g as a beta(+)- and gamma-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 Co-58m and Co-58g using a low-energy cyclotron. Also, the cellular S-values for Co-58m have been calculated and are presented here for the first time. Methods: Co-58m was produced via the Fe-58(p,n)Co-58m nuclear reaction on highly enriched Fe-58 metal. In addition, radiochemical separations of produced radio-cobalt from Fe-nat target material were performed. The theoretical subcellular dosimetry calculations for Co-58m and Co-58g 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 Co-58m was produced in the irradiation corresponding to a production yield of 10.7 MBq/mu Ah. The activity of Co-58g was measured to be 0.85% +/- 0.04% of the produced Co-58m 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 Co-58g, the TND and the potency of the Co-58m decrease with time after EOB. If a future treatment with a Co-58m labeled compound is not initiated before, e.g., 21 h after EOB, the resulting TND will be approximately 50% of the TND of 'pure' Co-58m as a result of the increased normal tissue dose from the ground state. Conclusions: The Auger emitter Co-58m 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 Co-58g requires fast radiochemical processing and use of future Co-58m-labeled radiopharmaceuticals in order to exploit the high achievable TND of Co-58m. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3608905]