Radiopharmaceutical Production Radionuclidic identity STOP

Radionuclidic identity The radionuclidic identity and purity tests show that the radiopharmaceutical does not contain radionculides other than the one desired. Contents Requirements Discussion Procedure

Requirements Requirement: The measured physical half-life of the test sample must be between 105 and 115 minutes. The test should be completed prior to release of product. The gamma spectrum of the test sample should show the major peak at 511 KeV, and a sum peak at 1022 KeV depending on geometry and detector efficiency. Not less than 99% of the gamma emissions should correspond to 18F. This test should be performed periodically.

Discussion Discussion: Half-life: Half life can be determined within the acceptable limits using counting equipment, such as dose calibrator or a well counter by measuring radioactivity of the test sample at two or more time points, and the decay calculations. In practical consideration for the short half-life of 18F and need to release the product the soonest, a precisely measured counting time of at least 10 minutes is generally sufficient to determine the physical half-life of 18F. The measured half life will be lower if 13N impurity is present in FDG. During validation studies, the time interval between measurements may be increased for improved assurance of radionuclidic purity. Gamma Spectrum: It must be realized that the mere presence of 511 KeV or 1022 KeV peak in γ-ray spectrum is not sufficient to determine radionuclidic identity. Impurity such as 13N (arising from 16O impurity in the target) and/or other positron emitters will not be detected as the gamma peaks with energy of 511 KeV and 1022 KeV sum peak are a common feature of positron emitters. Therefore, a combination of gamma spectrum and the half-life measurement together provide the best assurance of radionuclidic identity and purity.

Half-Life Procedure Procedure: For half-life measurement, place a small aliquot of the test sample in a dose calibrator or in a well counter. Record the initial radioactivity (A0). Record the radioactivity again after at least 10 minutes (measured precisely) (A10). Calculate the half- life from the two measured values as per the formula: T1/2 = 0.693 x t / [2.03 x [Log A0-Log A10]: where T1/2 and t are in minutes. Record the gamma spectrum (NaI or HPGe) of a test sample that has been diluted appropriately (and quantitatively) to provide optimum number of counts.

Half-Life Procedure Demonstration Procedure – In this demonstration, the test is done using a well counter and software written to calculate the half-life. A sample taken from the final product containing between 10,000 – 300,000 cpm. This particular program will record the number of counts in the sample 100 times, and run an algorithm that determines the half-life of the sample. It reports the half-life and the percent deviation. Half-life Values If the half-life values are not within the range specified in the Table, a common cause is that the amount radionuclide used was too high or too low, so this should be taken into account when repeating the determination. As an example, if the analysis gives an incorrect half-life, the amount of fluorine-18 sample should be reduced by a factor of 5 and the test repeated Radionuclide Half-life Acceptance Range Nitrogen-13 9 – 11 min Carbon-11 18 - 22 min Fluorine-18 105 - 115 min Demonstration

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