This feasibility study demonstrates (90)Y quantitative bremsstrahlung imaging of patients undergoing high-dose myeloablative (90)Y-ibritumomab treatment. METHODS The study includes pretherapy (111)In SPECT/CT and planar whole-body (WB) imaging at 7 d and therapy (90)Y SPECT/CT at 6 d and (90)Y WB imaging at 1 d. Time-activity curves and organ-absorbed doses derived from (90)Y SPECT images were compared with pretherapy (111)In estimates. Organ activities derived from (90)Y WB images at the first day were compared with corresponding pretherapy estimates. RESULTS Pretherapy (111)In images from 3 patients were similar to the (90)Y images. Differences between absorbed-dose estimates from pretherapy (111)In and (90)Y therapy were within 25%, except for the lungs. Corresponding activity differences derived from WB images were within 25%. Differences were ascribed to incomplete compensation methods and real differences in pharmacokinetics between pretherapy and therapy. CONCLUSIONS Quantitative bremsstrahlung imaging to estimate organ activities and absorbed doses is feasible.