Assessment of radionuclidic impurities in 2-[18F]fluoro-2-deoxy- D-glucose ([18F]FDG) synthesis with the GE FASTlab synthesizer

Fast and automated synthesis of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), the most important radiopharmaceutical used in positron emission tomography, has become, in recent years, a main requirement for hospitals and pharmaceutical industries. During the production of fluorine-18 in a cyclotron by the 18O(p,n)18F reaction, impurities can be produced by proton and secondary particles that hit materials surrounding the target of enriched [18O]water. The aim of this study was to assess the radionuclidic impurities deposited, during the synthesis of [18F]FDG, in the FDG cassette of the GE FASTlab synthesizer and the characterization of the cassette as radioactive waste. [18F]fluoride was produced using a GE PETtrace cyclotron able to accelerate protons up to 16.5 MeV. The target system used was the standard GE high-yield assembly comprised of a silver chamber filled with [18O]water at a pressure of 30 bar (450 psi). The front of the target body is sealed with a 25 mm HAVAR foil. Typical production irradiations considered for this work were 100-120 min at a current of 35-38 mA with an [18F]fluoride activity, produced at the end of bombardment, of 168 GBq (about 4600 mCi). The four cartridges used in the FDG cassette (QMA, tC18, Alumina, and OASIS HLB ®) and the reactor vessel were systematically measured in a gammaray spectrometer after a waiting time of 24-72 hours from the end of synthesis to allow a sufficient decrease in radioactivity to bring the total counting frequency to less than 2000 counts per second in the 10-2000 keV energy range. A high-purity germanium detector, N-type, with a 15% relative efficiency and a resolution of 1.8 keV at 1332 keV was used. An appropriate library was created for the spectra analysis on the basis of the knowledge of the irradiated materials: (p,n), (p,d), (p,a), (n,g) reactions on HAVAR, target body and silver chamber were taken into account to create a table of the possible radionuclides present in the samples measured. Samples were measured for 1800 seconds each. About 30 FDG cassettes were measured and 150 spectra analyzed. All the significant peaks found in the spectra analyzed were identified. The radionuclides found were: 26Al, 51Cr, 52Mn, 56Co, 57Co, 58Co, 95Tc, 96Tc, 181W, 181Re, 183Re for the QMA cartridge; 51Cr for the tC18 cartridge; 51Cr for the Alumina cartridge; 51Cr and 96Tc for the OASIS cartridge; 51Cr, 96Tc for the reactor vessel. The mean activities found in the cartridges were: 1.37E+03 ± 8.48E+02 Bq for the QMA; 1.41E+01 ± 5.17 Bq for the tC18; 1.13E+01 ± 6.91 Bq for the OASIS; 2.98E+01 ± 1.36E+01 for the Alumina, and 2.03E+01 ± 8.63 for the reactor vessel. The average activity found per cassette was 1.40E+03 ± 8.67E+02. In conclusion, an extensive measurement campaign on the FDG cassette was conducted. [18F]fluoride from the cyclotron goes through the QMA cartridge first and it is here that most impurities remain trapped. The activity level in the other cartridges was less than that in the QMA. Traces of impurities were also found in the reactor vessel because that is the place where synthesis reactions take place. The activity level of the cassette does not modify the classification of the area or the risk for radiochemistry laboratory personnel, but the presence of radionuclides with a half-life greater than 75 days means that the cassettes constitute radioactive waste and must be be treated in accordance with the relevant Italian law (D.Lgs. 230/95).