177 Considerations on bone volume normalization in quantifying skeletal burden in fibrous dysplasia material is frequently encountered in FD patients, needing exclusion frommeasuring SV. In our cohort, cortical bone was therefore segmented by setting an upper threshold of + 2500 HU, which visually succeeded in excluding osteosynthesis material without excluding bone. This resulted in a mean SV of 6.4 L (range 4.3 to 9.2 L). Although this mean SV value is in accordance with Duarte and Sapienza’s findings [1], we found a considerable larger inter-individual range. This might be explained by heights of our patients ranging from 1.47 to 1.87 m (adults only) and possibly the extent of skeletal FD involvement in our cohort. Furthermore, we expected that SV would be independent of the other investigated measures of distribution volume BW and LBM. We did, however, measure a significant positive correlation between these parameters (SV versus BW: r = 0.62; p = 0.002 and SV versus LBM: r = 0.83; p < 0.001) which suggest that, at least in our cohort of adult patients, patients with higher (lean) body mass have a higher skeletal volume. We then converted BW-normalized HB SUVs to SV normalized SUV-SVs, using the equation SUV-SV = (SUV/ BW) × SV, and examined the independency of SUV-SV with respect to SV. The correlation between HB mean SUV-SV and its normalization factor for volume of distribution SV was significantly positive (r = 0.51; p = 0.012), and therefore, in our cohort, normalization by SV seems to perform equally bad to normalization by LBM (r = 0.53; p = 0.009) and seems to perform worse than normalization by BW (r = 0.37; p = 0.057). The authors compared HB SUV-SVs and SUVs between patients of low (< 50 kg) and high (> 90 kg) BW and found no statistical significant difference in mean HB max SUV-SV in proximal diaphysis of both the right humerus (unpaired t test; p = 0.87) and right femur (p = 0.27) [5]. The correlation between HB mean SUV-SV and BW in our cohort, however, was significantly positive (r = 0.62; p = 0.002), implying that higher HB SUV-SVs are found in heavier patients, which is in firm contrast with the authors’ observation. Moreover, the individual patient cutoff SUV-SV for HB still showed a large range above the predefined 10% of its median value, and therefore, according to our previous definition, a per scan HB still needs to be measured (mean SUVSV cutoff 0.51 L/mL, range 0.28 to 0.72 L/mL; mean absolute variability 10%, range 2–45%). In the study by Gomes Marin, et al., the respondents found a mean absolute variability of the HB SUV-SV of 13% (range 0–47%) [6], which is almost identical to our findings. The mean absolute variability in HB SUV was somewhat higher than that in HB SUV-SV in their cohort (16%, range 0–62%); likewise, the variability in SUV was somewhat higher in 7