Long lived radionuclidic impurities in FASTlab F-18 FDG citrate

Objectives Previously published long lived radionuclidic impurity analyses were from silver targets with havar foils at low current and using older FDG synthesis modules. The authors believe this to be the first report of long lived radionulcidic impurities on the FASTlab FDG synthesis unit using high yield F-18 niobium targets with havar foils.

Methods A total of 3 FDG lots produced using the FASTlab FDG citrate cassette were analyzed after a 14 day decay period. Approximately 12Ci of F-18 fluoride was produced by dual target bombardment of >97% H218O at 50µA per target for 3 hrs. Standard FDG syntheses were conducted and all specifications were met for clinical use. At 14 days, a 1mL aliquot of the FDG was counted on a high purity germanium spectrometer for 2 hours. Non-background photopeaks in the spectra were identified, integrated and converted to absolute radioactivity units. The measured activities were corrected to FDG lot expiration, 12 hours post end of synthesis.

Results The following radionuclides were detected in each lot: Co-56, Co-57, Co-58, Cr-51, Mn-54, Tc-95m, Re-183 and Re-184. The largest average radionuclidic impurity was Re-183 (187nCi), with the average sum of radionuclidic impurities of 348nCi per lot, an average of 0.041% contaminant level at lot expiration.

Conclusions The work presented here shows a higher detected level of long-lived radionuclidic impurities in the FDG formulation than previously reported. Some shorter lived impurities may have been missed due to the 14 day decay. The reported level is well below NRC and USP regulatory limits for long lived radionuclidic impurities. The higher level of impurities is postulated to originate from the elution of QMA SPE, as the FASTlab, unlike previously reported systems, elutes the QMA using a flow in the opposite direction from F18 loading. This would allow impurities trapped at the top of the QMA to be carried into the FDG synthesis. The long lived radionuclidic impurities are consistent with being produced in the havar foils during bombardment.