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Background and Purpose We compared [18F]fluoro-2-deoxy-2-D-glucose (FDG) versus 3′-deoxy-3′-[18F]fluorothymidine (FLT) for the purpose of identifying active pelvic bone marrow (BM), quantifying its locational variation, and determining which technique is likely to be better for BM-sparing radiation planning. Material and Methods We sampled 41 patients, of which 25 underwent FDG-PET/CT only, 7 underwent FLT-PET/CT only, and 9 underwent both. Active BM subvolumes were defined as subsets of the pelvic BM with the highest standardized uptake values comprising 40%, 50%, and 60% of the total pelvic BM volume. We used the Dice similarity coefficient to quantify the percent overlap of active BM volumes of equal size. Differences in the spatial distribution of active BM were assessed using a region-growing algorithm. Results For patients with both modalities, the mean Dice coefficients for the 40%, 50%, and 60% subvolumes were 0.683, 0.732, and 0.781 respectively. Comparing individual active BM subvolumes to the mean subvolume, Dice coefficients varied from 0.598–0.889 for FDG and 0.739–0.912 for FLT. Region growing analysis showed FLT-PET defined more highly clustered active BM subvolumes. Conclusions Within the limitations of a small sample size, we found significant agreement between FDG-PET and FLT-PET; however, FLT-PET had significantly less individual variation and is likely to be superior to FDG-PET for BM-sparing radiotherapy.

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