A comparison of methods for adapting 177 Lu dose-voxel-kernels to tissue inhomogeneities

Purpose: In 177 Lu radionuclidetherapies,dosimetry is used for determining patient-individual dose burden. Standard approaches provide whole organ doses only. For assessing dose heterogeneity inside organs, voxel-wise dosimetry based on 3-D SPECT/CT imaging could be applied. Often, this is achieved by convolving time activity-curves with appropriate dose-voxel-kernels (DVK) voxel-wise. Theexact shape of DVKs depends on type and density of the tissues under consideration. In literature, DVKs are often not adapted to these inhomogeneities, or simple approximation schemes are applied. Furthermore, in many studies the number of evaluated patients has been very low. Consequently, the aim of this study was to compare different methods for adapting DVKs in a large number of patients. Methods: In 26 patients, which had previvously undergone a Lu-177-PSMA or -DOTATOC therapy, activitymaps, mass-density, and tissue-type were derived from SPECT/CT acquisitions. These were used for a voxel-based dosimetry based on convolution with DVKs (each of size 4.8 × 4.8 × 4.8 mm3) obtained by four different methods proposed in literature (herein named 'constant', 'center-voxel', 'density', 'percentage'). Deviations between resulting dose values and those from full Monte-Carlo simulations (MC simulations) were compared for selected organs and tissue-types. Results: For each method, inter-patient variability was considerable. In kidneys and spleen, 'constant' and 'density' achieved results with smallest deviations (~ 5 to 8 % underestimation). For lung, adipose, muscle, and bone tissue, the methods 'centervoxel', 'percentage', and 'constant' achieved superior results, respectively. Conclusion: In this currently largest study on the topic, no single DVK-adaption method was consistently better than any other. The high inter-patient variability indicates that for evaluating new algorithms a sufficiently large patient cohort should be used.