[Influence of radioactive concentration and storage time on radiochemical purity of 18F-FDG]

To study the influence of the 18F-FDG radioactive concentration and the usual greatest storage time of the radiopharmaceutical at the Radiopharmacy Unit (RU) over its radiochemical purity.

Thirty 18F-FDG preparations coming from different batches were studied. The radiochemical purity was determined at the RU by means of TLC to saline-diluted (1:10) and undiluted samples of each preparation, in the early 30 minutes since its arrival and 5 hours later. The radiochemical purity of the original 18F-FDG was determined at the PET radiopharmaceutical producer Laboratory (PETL) by means of HPLC in the early hour since the 18F-FDG dispensing.

The increase of 18F-Fluoride found in the (5 h-30 min) period was significantly greater in the samples without diluting than in the diluted ones (p < 0.0001). We found a significant correlation between the percent of this increase of 18F-Fluoride (y) and the radioactive concentration of the 18F-FDG (x): y = 0.00061x + 0.1759 (R2 = 0.198; p < 0.0005). The percent of 18F-Fluoride determined at the RU was significantly higher than the percent of 18F-Fluoride determined at the PETL (p < 0.0001). A significant correlation between the differences of the percent of 18F-Fluoride determined by TLC and HPLC (y) and the radioactive concentration (x) was found: y = 0.0139x + 0.3146 (R2 = 0.196; p = 0.016). A significant correlation among the differences of percent 18F-Fluoride determined by TLC and HPLC ([%F] RU - [%F] PETL), the radioactive concentration (RC) and the time since the radiopharmaceutical dispensing (t) was found: [%F] RU - [%F] PETL = 0.01159*RC (mCi/mL) + 0.250*t (h) - 0.01903 (R2 = 0.226; p < 0.014).

The stability of the 18F-FDG preparations with time increases when diminishing its concentration. We recommended the dilution of these preparations with physiological saline solution.