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 Y-90 glass and resin micro -spheres were found. In the present work, the origin of these discrepan-cies was investigated by Monte Carlo (MC) simulations. Methods: Three vial configurations, containing Y-90-chloride, Y-90-labeled glass microspheres, and Y-90-labeled resin microspheres, were modeled with GAMOS, and the electric signal generated in an activity meter was simu-lated. Energy deposition was scored in the activity meter-active regions and converted into electric current per unit activity. Internal bremsstrah-lung (IB) photons, always accompanying 0 -decay, were simulated in addition to Y-90 decays. The electric current per source activity obtained for Y-90 glass and resin microspheres, Iglass and Iresin, was compared in terms of relative percentage difference with that of Y-90-chloride (sglass and sresin) and each other (8). 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 sglass and sresin results were 24.6% +/- 3.9% and-15.0% +/- 2.2%, respectively, whereas 8 was 46.5% +/- 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 Y-90 glass and resin microspheres can be attributed to differences in the geometry of the respective commer-cial vials and to the metrologic approach adopted for activity meter cali-bration with a Y-90-chloride liquid source. Furthermore, IB photons were shown to play a relevant role in determining the electric current in the activity meter.