169 Quantifying skeletal burden in fibrous dysplasia using Sodium Fluoride PET/CT no relationship with SF-36. These findings are in accordance with previous reports that QoL and pain are not related to disease burden [2, 26]. Although our cohort displays higher SBS in assisted patients and this is in accordance with previous findings by Collins, et al., we are cautious to draw any clinical conclusions [2], since impaired mobility is a multifactorial parameter and, in our experience, is predominantly determined by a combination of the FD localization (mainly proximal femur lesions), use of analgesic medication and pathological fractures. These factors are not encompassed in the SBS and therefore it will be difficult to claim a direct causal relation between SBS (or Na[18F]F-PET/CT) and mobility. Additionally, as pathological fractures will mostly arise in weight-bearing bones, it may be of interest to further subdivide Na[18F]F-PET/CT findings in FD-patients into pathological uptake in weight-bearing and non-weight-bearing bones in case of fracture risk analysis. When comparing our results to the very recent publication by Papadakis, et al. in a slightly smaller cohort of 15 patients, we see a congruent result on the correlation between Na[18F]F-PET/CT (volume-based) parameters defining FD with serum ALP [11]. No correlation between Na18F-PET and the bone resorption marker CTX was measured, where they did find a relation with N-telopeptides [11]. The correlation with both P1NP and FGF-23 were never investigated by another group and this adds new clinical insights. Our study propagates per patient individualized determination of healthy bone on Na[18F]F-PET/CT and specific exclusion of Na18F-uptake caused by OA. Finally, our data shows higher TLF on Na[18F]F-PET/CT in baseline patients using bisphosphonates and at the same time confirms the previous finding by Collins, et al. in 2005 of independence of Skeletal Burden Score in patients ‘on or off bisphosphonate’, but a larger sample size is warranted to confirm our current findings [2]. A limitation is that the currently used VOIs in accordance with Rohren, et al. are relatively small, comprising only around 8 voxels per VOI and are by definition representing trabecular (not cortical) bone only [6]. Development of a method to measure normal bone turnover in both trabecular and cortical bone, using larger VOIs, less in number, could be considered in future studies, as this may theoretically further improve clinical applicability of the technique. Also, as our normal bone measurements comprised bones that are weight bearing, it remains debatable whether physiological bone metabolism determined in bones that are not or hardly influenced by body weight (e.g. humerus or skull) would reach the same result. We analyzed SUVmean values instead of the theoretically superior SUVmedian, although the distribution of Na18F uptake in a (manual) VOI is slightly positively-skewed, partially due to the non-negativity constraint in iterative reconstruction algorithms. In positively-skewed distributions the measure ‘mean’ is an overestimation of ‘central 6