Monte Carlo Simulations Corroborate PET-Measured Discrepancies in Activity Assessments of Commercial 90Y Vials

In a recent multicenter study, discrepancies between PET/CT-measured activity and vendor-calibrated activity for 90Y glass and resin microspheres were found. In the present work, the origin of these discrepancies was investigated by Monte Carlo (MC) simulations. Methods: Three vial configurations, containing 90Y-chloride, 90Y-labeled glass microspheres, and 90Y-labeled resin microspheres, were modeled with GAMOS, and the electric signal generated in an activity meter was simulated. Energy deposition was scored in the activity meter–active regions and converted into electric current per unit activity. Internal bremsstrahlung (IB) photons, always accompanying b-decay, were simulated in addition to 90Y decays. The electric current per source activity obtained for 90Y glass and resin microspheres, Iglass and Iresin, was compared in terms of relative percentage difference with that of 90Y-chloride («glass and «resin) and each other (d). The findings of this work were compared with the ones obtained through PET measurements in the multicenter study. Results: With the inclusion of IB photons as primary particles in MC simulations, the «glass and «resin results were 24.6% 6 3.9% and 215.0% 6 2.2%, respectively, whereas d was 46.5% 6 1.9%, in very good agreement with the values reported in the multicenter study. Conclusion: The MC simulations performed in this study indicate that the discrepancies recently found between PET/CT-measured activity and vendor-calibrated activity for 90Y glass and resin microspheres can be attributed to differences in the geometry of the respective commercial vials and to the metrologic approach adopted for activity meter calibration with a 90Y-chloride liquid source. Furthermore, IB photons were shown to play a relevant role in determining the electric current in the activity meter.