Abstract
1424
Objectives To compare voxel-based dose calculation methods for Y-90 PET using a commercially available software package. Comparison is made to doses calculated from a mathematical phantom and to doses calculated using the MIRD schema with the known injected activity.
Methods A torso phantom with 0.74 GBq of Y-90 in the liver and 60 mL tumor (5:1 tumor-to-liver) was imaged with a Siemens Biograph mCT PET/CT. Images were acquired over 90min so that the total counts would be similar to a 20min patient scan with 4 times the activity, which is typical for radioembolization with Y-90 microspheres. The PET image was reconstructed using a 200 x 200 matrix and 3D OSEM (1 iteration, 21 subsets) with TOF, which was found to have the best quantitative accuracy in a previous work (Dewaraja et al. EJNMMI (2015) 42 Suppl 1:S156). Liver and tumor were segmented from the CT scan. Local Deposition Method (LDM), LDM with scaling for “known” injected activity, and a Dose Point Kernel (DPK) method using the MIRD kernel were used to perform 3-D dosimetry on PET. For comparison, DPK estimates of “true” dose were obtained for a mathematical phantom created by masking the compartments of the physical phantom to known activity levels. Comparisons were also made to the mean dose values from hand calculations using the simplified MIRD formula for Y-90 (Dose[Gy]=49.67[asterisk]Activity[GBq]/mass[kg]).
Results LDM with scaling showed deviations of -0.5% and 5.5% for non-tumoral liver and tumor respectively compared to the mean dose from the mathematical phantom. Using the PET Bq/mL values directly without scaling, LDM showed deviations of -18.9% and -14.8% for non-tumoral liver and tumor while DPK showed deviations of -16.5% and -14.2% for non-tumoral liver and tumor respectively. The dose to the non-tumoral liver and tumor from the MIRD calculation was 21.8 Gy and 109.6 Gy respectively.
Conclusions PET-based dose estimates using LDM with scaling compared closely with the expected doses. Without scaling by injected activity, both LDM and DPK methods performed similarly with deviations reflective of the quantitative accuracy of Y-90 PET. Scaling for the injected activity appeared to better account for the quantitative accuracy of Y-90 PET.