Activity measurements of 18F and 90Y with commercial radionuclide calibrators for nuclear medicine in Switzerland

The activity of radiopharmaceuticals in nuclear medicine is measured before patient injection with radionuclide calibrators. In Switzerland, the general requirements for quality controls are defined in a federal ordinance and a directive of the Federal Office of Metrology (METAS) which require each instrument to be verified. A set of three gamma sources (Co-57, Cs-137 and Co-60) is used to verify the response of radionuclide calibrators in the gamma energy range of their use. A beta source, a mixture of 90Sr and 90Y in secular equilibrium, is used as well. Manufacturers are responsible for the calibration factors. The main goal of the study was to monitor the validity of the calibration factors by using two sources: a 90Sr/90Y source and a 18F source. The three types of commercial radionuclide calibrators tested do not have a calibration factor for the mixture but only for 90Y. Activity measurements of a 90Sr/90Y source with the 90Y calibration factor are performed in order to correct for the extra-contribution of 90Sr. The value of the correction factor was found to be 1.113 whereas Monte Carlo simulations of the radionuclide calibrators estimate the correction factor to be 1.117. Measurements with 18F sources in a specific geometry are also performed. Since this radionuclide is widely used in Swiss hospitals equipped with PET and PET-CT, the metrology of the 18F is very important. The 18F response normalized to the 137Cs response shows that the difference with a reference value does not exceed 3% for the three types of radionuclide calibrators.