Development of a gamma counter calibration protocol for radiopharmaceutical applications

Objective
In the context of the implementation of specific clinical trial protocols involving new radionuclides or for routine use needing, calibration of the gamma counter is required. The aim of this study is to determine the counting efficiency for several isotopes and to develop a standardized and reproducible protocol to facilitate the metrological and the analytical validation needed.

Materials and methods
After defining the operational framework for the use of the gamma counter, the critical parameters for sample analysis (such as volume, counting time, etc.) were determined. A protocol was developed and an Excel spreadsheet was created to calculate the required dilutions to cover the analytical range. Qualification of the dose calibrator, analytical balance and micropipettes were performed to ensure the reliability of the results. First, we determined the maximum volumetric activity before saturation of the gamma counter (determined with a dead time factor < 1,04). To perform this step, we started from a radioactive stock solution obtained from a known volumetric activity solution of each radionuclides required. The volumetric activities were determined by dose calibrator. Dilutions of these stocks solutions were made and the dead time were measured. These initial results allowed the creation of a second calibration range with volumetric activities under this saturation limit, which included the previously established volumetric activities, adapted for routine analyses. The theoretical number of CPM (Count Per Minute) calculated for each dilution was compared to the practical CPM obtained by gamma counter measurement. The counting efficiency for the studied isotope was determined, and an accuracy profile of ± 10% was established for the measurements.

Results
The creation and subsequent improvement of the measurement protocol and Excel file to standardize the determination of counting efficiency was used for routine ^99mTc, ¹²⁵I or therapeutic ¹⁷⁷Lu isotope and for PET emitter such ⁸⁹Zr. The counting efficiency for ^99mTc, ¹²⁵I, ¹⁷⁷Lu and ⁸⁹Zr were respectively 79.5%, 100.0%, 9.2% an 23.0%. The accuracy profiles permit to define the analytical range of values where the error percentage on the counting efficiency is ± 10%.

Discussion/conclusion
The determination of counting efficiency is an essential metrologic step for the analysis of gamma counter results. It’s important to note that the geometry of the sample (volume, shape and nature of the vial) may impact these results. Consequently, it appears necessary to repeat the study if measurements need to be performed with different parameters. This protocol formalization is now implemented in our hospital radiopharmacy unit and serve as a framework for a simpler implementation of new radionuclides in nuclear medicine.

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