Evaluation of quantitative 90Y bremsstrahlung SPECT based on patient studies

378 Objectives Several studies have shown that it is possible to image 90Y, by imaging the produced bremsstrahlung. Many of these studies have focused on the possibility of activity quantification by means of phantom studies. In this study we have focused on evaluating quantitative 90Y SPECT using patient measurements. Measurements were conducted on patients undergoing high dose treatment with 90Y-Zevalin. The patients participated in a dose escalation study which included a pre-therapy study with 111In-Zevalin. The absorbed dose to the liver was estimated and used to determine the activity to be administered for therapy to obtain an absorbed dose to the liver of 12 Gy. The 90Y images were evaluated qualitatively by comparing them with the 111In images and the dose to the liver was compared to the pre therapy estimate. Methods Projection data were acquired using a GE VH/Hawkeye system. Seven time points were measured during pre-therapy and six time points during therapy. Transversal images were reconstructed using an OSEM algorithm with attenuation, scatter and collimator response (CRF) compensation. The scatter and CRF compensations require pre-generated kernels, which were generated with the SIMIND MC code. The camera sensitivity for 90Y was measured using a known activity in a Petri dish placed under 16 mm Lucite. The 111In sensitivity was measured in air. Results Two patients have been evaluated and in both cases the 90Y images show a clear similarity to the 111In images. The liver doses were estimated to 12.1 Gy and 14.5 Gy. The 20% discrepancy for one patient was determined to be a result of differences in kinetics between the pre therapy and therapy studies Conclusions The results are promising and show that it is possible to omit 111In-mAb for imaging during therapy.