Identification and quantification of radioactive impurities in medical radionuclides

The presence of radioactive impurities in radionuclides that are used for medical applications may adversely affect the quality of diagnostic images and increase radiation dose to the patient. The aim of this project was to identify and quantify such impurities in the following solutions: 99mTc generator eluates, commercial 123I and 131I sodium iodide and cyclotron-produced 68Ga and 89Zr. The work was done with a calibrated high-purity germanium (HPGe) detector and gamma spectroscopy system with the activity of the main radionuclides being measured with a radionuclide calibrator. Impurity activity was expressed as a percentage of the main radionuclide activity. The parent 99Mo was found in all the 99mTc generator eluates but with low relative activity (<3x10-3%). There was no significant difference between the corresponding eluates of two generators having different reference activity. Overall, the relative activity decreased logarithmically with eluate number and this has not been reported previously. This suggests that as regards 99Mo content, the later eluates are safer for patients than the earlier ones because they deliver lower additional radiation dose. The radionuclide 103Ru was also identified as an impurity in some 99mTc generator eluates, but at a much lower relative activity (<2x10-5%). This decreased rapidly after the first elution suggesting that, in effect, its presence may be eliminated by pre-elution of the generator. For the Na123I solution, the main contaminants were 125I (<2x10-2%) and 121Te (<7x10- 3%). However, also observed were trace activities (<8x10-4%) of 124I, 126I, 95mTc and 96Tc, which have not been reported in the literature. As these impurities were found only in some samples with very low relative activity and as their half-lives are similar to or shorter than the half-life of the longer-lived main impurity (125I), they do not present a significant hazard to patients. No impurities were identified in the Na131I solution. The radionuclides 68Ga and 89Zr were produced by a cyclotron at the Cardiff University Positron Emission Tomography Imaging Centre (PETIC), the former for the first time in the UK using a liquid target. For 68Ga, the identified impurities were 67Ga (<9x10- 1%), 66Ga (<7x10-3%) and 56Co (<5x10-4%), while for 89Zr, the contaminants were 88Zr IV (<2x10-3%), 88Y (4x10-4%) and 56Co (<3x10-3%). In both cases, the relative activities of the impurities were well within the goal of