157 Quantifying skeletal burden in fibrous dysplasia using Sodium Fluoride PET/CT treatment and based on current literature postulated by Collins, et al., this would not have had an effect on the bone scintigraphy findings and/or SBS [2]. Any increased uptake on Na[18F]F-PET/CT, defined as voxels with Na18F-uptake higher than the surrounding normal bone at sites of suspected FD, was assessed in conjunction with the low-dose CT to be based on FD. Other sites with an increased accumulation of Na18F higher than the cut-off but not attributable to FD were excluded, assessing the Na18F-PET-images in conjunction with integrated low-dose CT images to conclude osteoarthritis, fracture or osteosynthesis material. FD skeletal burden was quantified using four parameters: (1) maximum standardized uptake (denoted with SUVmax), (2) the peak standardized uptake in a 1-cm3 sphere (denoted with SUVpeak), (3) the Fluoride Tumor Volume (FTV), defined as the volume of Na18F-uptake higher than cut-off and attributable to FD, and (4) the Total Lesion Fluoride uptake (TLF), being the product between FTV and itsmean standardized Na18F uptake. The TLF therefore is analogous to Total Lesion Glycolysis in 18FDG-PET/CT [20, 21]. A high correlation between SUVmax and SUVpeak as well as between the volumetric Na18F-parameters TLF and FTV was expected. We predefined that in case the explained variance between these parameters was very high (R2>0.95), SUV max and FTV would be excluded from further analysis, as SUVpeak is more stable to image noise than SUVmax and TLF characterizes both volume and uptake of FD [22]. Clinical and biochemical parameters The following clinical items were collected from the electronic patient files: FD localizations, active therapy of antiresorptives interfering with bone metabolism (both available for n=20). Actual mobility (defined as either unassisted, crutches or in a wheelchair) was collected in accordance with Collins, et al., yet we recognize that this parameter is often mainly influenced by other clinical factors, such as location of the FD-lesions, stress fractures and use of analgesic medication. Second, pain assessment using a validated structured clinical pain score (Brief Pain Inventory, BPI, n=15) and the validated quality of life questionnaire SF-36 n=10) were taken within a timeframe of two to seven weeks to the time of the Na[18F]F-PET/CT [14, 15]. According to the analytical methods previously described the following baseline markers of bone turnover (BTM’s) were determined: ALP, P1NP, CTX (n=20 for all these biomarkers) and FGF-23 (n=16) [23]. Statistical analysis All variables were assessed for (log)normality based on skewness and kurtosis metrics. In case of (log)normality, parameters were described as mean ± SD and 6