MO‐G‐BRA‐02: Microdosimetry of Simulated Heterogeneous 177Lu and 90Y Activity Distributions

Purpose: The aim of this study was to determine the dosimetric impact of heterogeneous cellular uptake of Lu‐177 and Y‐90 labelled compounds in targeted radionuclide therapy (TRT). Methods: Fine resolution voxel S values (VSVs) were generated for Lu‐177 and Y‐90 using Monte Carlo simulation with the EGSnrc user‐code DOSXYZnrc. The VSVs were generated to be 20 microns on edge for Lu‐177 and 100 microns for Y‐90. To simulate varying degrees of heterogeneous uptake, activity was randomly assigned to percentages ranging from 1% to 80% of voxels in cubic volumes chosen to be large enough so that a central region, 30×30×30 voxels in size could receive a cross dose from the maximum beta range of Lu‐177 and Y‐ 90. The resulting dose distributions were determined using the VSV approach where each target voxel dose is calculated by summing the product of the activity and corresponding voxel‐to‐voxel S value from each source voxel. Results: For the simulated Y‐90 activity distribution, when 80% of the 100 micron voxels were labelled, the minimum and maximum voxel doses in the central region of interest were within 2.2% and 1.3% of the mean voxel dose, respectively. A percent difference of less than 10% was maintained between the minimum/maximum and mean voxel dose all the way down to 20% labelling. At 80% labelling of Lu‐177, the minimum and maximum voxel doses were within 11.1% and 5.4% of the mean voxel dose, respectively, and at 20% labelling these values increased to 21% and 40%. Conclusions: This study indicates the impact of nonuniform activity distributions of two commonly used radionuclides in TRT. Even with low percentage labelling, the relatively long range of Y‐90 emissions helps to significantly smooth out the dose. The calculated dose distributions were not as uniform for Lu‐177 due to the shorter range of its emitted betas.