Influence of radiation source geometry on determination of (111)In and (90)Y activity of radiopharmaceuticals.

Objective Yttrium-90 (Y-90)-labelled peptides such as DOTATOC and antibodies such as Zevalin are widely used in radionuclide therapy. Indium-111 (In-111) is used as a Y-90 surrogate for imaging and dosimetry purposes. We aimed to investigate accuracy, geometry (vials and syringes) and volume dependencies for both radionuclides in several different radionuclide calibrators. Methods (YCl3)-Y-90 and (InCl3)-In-111 solutions were gravimetrically dispensed into the most frequently used containers. In each container several dilutions of the parent solutions were performed. Mass, activity and time were recorded for each calibrator and measurement. Aliquots of both parent solutions were calibrated at the National Metrology Laboratory, Vienna, Austria (BEV). From our measurements and results from BEV, correction factors were determined and further partitioned into calibration, geometry and volume correction factors. Results Using the nominal calibration factors provided by the manufacturer, measured activity in PIS vials was overestimated by up to 25% for In-111, depending on the calibrator. Y-90 activity was either underestimated (by up to 20%) or overestimated (by up to 25%) using different radionuclide calibrators. This is the result of the difference in containers used to set the manufacturer's calibration factor values and the containers used in nuclear medicine departments and in this study. There was little geometry dependence for glass vials but strong geometry dependence for syringes for both radionuclides in all calibrators. Conclusion The results should constitute a warning for all personnel responsible for preparation of radiopharmaceuticals. Every nuclear medicine department should incorporate a proper quality-control regimen for radionuclide calibrators and a quality-assurance system. Nucl Med Commun 30: 807-814 (C) 2009 Wolters Kluwer Health vertical bar Lippincott Williams & Wilkins.