Impact of vial geometry on ¹⁸F radionuclide calibrator response using ⁶⁸Ge/⁶⁸Ga surrogate sources

Accurate quantification of radiopharmaceutical activity is essential for reliable PET imaging, particularly in determining Standardized Uptake Values (SUV). Radionuclide calibrators used in radiopharmacy are typically factory-calibrated using standard reference geometries, which often differ from those employed in routine clinical practice. This discrepancy can introduce systematic errors unless geometry-specific correction factors are applied. In this study, we investigated the use of sealed ⁶⁸Ge/⁶⁸Ga sources as a mock source for ¹⁸F, leveraging their emission of 511 keV photons. A literature-based calibration factor for converting ⁶⁸Ge/⁶⁸Ga readings to ¹⁸F (1.016) was applied. However, since the geometry of the sealed calibration source (plastic vial) differs from that of the GMP glass vials routinely used in Radiopharmacies, an additional geometry correction factor was experimentally determined. Two ¹⁸F sources with identical activity were prepared using gravimetric methods in plastic and glass containers. Measurements were performed across multiple Radionuclide calibrators. The correction factor was derived from the ratio of measured activities between the two vial types. The results showed a consistent geometry correction factor (mean 1.015; CV 0.37 %) across all radionuclide calibrators tested, indicating limited dependence on calibrator model. Additional tests performed with two ⁶⁸Ge/⁶⁸Ga sources of differing activities confirmed the feasibility of this approach, while also highlighting the importance of certified reference sources to minimize uncertainty. These findings support the implementation of sealed ⁶⁸Ge/⁶⁸Ga sources as long-lived references for ¹⁸F calibrator verification. When combined with geometry-specific correction factors, this approach offers a practical and reproducible alternative to conventional calibration procedures, particularly useful in GMP-regulated settings.