Intercomparison of activity measurements for beta-emitters in Swiss nuclear medicine laboratories

The activity of injected radiopharmaceuticals in nuclear medicine, including beta-emitters used for pain palliation, has to be monitored systematically. The objective of the present work was to evaluate the situation and precision of activity monitoring for beta-emitters in Swiss nuclear medicine laboratories.

A questionnaire about the monitoring methods used was sent to 50 centers. On the basis of the questionnaire results, an intercomparison of activity measurements with 90Y and 169Er sources was organized.

This study showed that most laboratories check beta-emitter activity with a dose calibrator measurement in the original vial provided by the producer or in the injection syringe. They therefore need to have calibration factors for the corresponding measurement geometries. The results of the intercomparison were disappointing overall. Sixteen of 27 90Y measurements and 17 of 22 169Er measurements in the original vial deviated from the reference activity by more than 20%. The situation was similar for the syringe. These discrepancies did not stem from the intrinsic limitation of the measuring method but were mainly attributable to the poor quality of the calibration factors provided by the manufacturers, in addition to lack of follow-up and incorrect background subtraction, particularly for 169Er, by the nuclear medicine laboratories. Manufacturers are being contacted to discuss possible improvements for the situation.

This study showed that commercial dose calibrators are generally adequate for measurement of the activities of beta-emitters. However, in some cases, the measurement of 90Y can lead to errors reaching +/-50%. For 169Er, with its much lower beta-energy, the situation is even worse; the observed differences can be higher than 1 order of magnitude.