155 Quantifying skeletal burden in fibrous dysplasia using Sodium Fluoride PET/CT Materials and methods Oral informed consent was obtained from all patients for the use of their imaging data and written approval for the retrospective analysis and reporting of the collected data was obtained from the Medical Ethics Committee of the LUMC. Population At the Center for Bone Quality of the LUMC, a national and European referral center for metabolic bone diseases including FD, all new patients that underwent a full body Na[18F]F-PET/CT scan performed for the baseline assessment of FD disease burden before start of denosumab treatment or planned surgery, between February 2015 and April 2018, were eligible. Scan protocol The PET/CT acquisition was preceded by standard clinical preparation with adequate hydration shortly before scanning and intravenously administration of a bodyweight (BW)-adjusted dose of Na18F and with one minute per bed position and 50% overlap (table 1). Attenuation and scatter-corrected total-body PET-images (Philips Gemini TF TOF 64T) in supine position were obtained in all patients resulting in 4 x 4 x 4 mm voxels. The PET-images were reconstructed using a time of flight reconstruction algorithm. Directly prior to PET-acquisition, integrated high-quality low-dose CT without (intravenous) contrast was acquired for attenuation correction and localization (settings: 120 kVP, mAs modulation (range 50 – 550 mAs; depending on body part and patient characteristics), slice thickness 0.9 mm, bone kernel, and slice spacing 0.9 mm, 768x768 matrix size (resulting in 0.46x0.46x0.9 mm voxels) and the images were interpreted in dedicated bone and soft tissue window in all 3 dimensions, bone window level/width 400/2400. Literature shows that both extent of FD, the relevant alternative diagnosis (osteoarthritis, multiple myeloma and other malignant bone disease) can be discerned on high-quality and thin sliced LDCT as used in our study and this supports our experience in practice [16-18]. Thus, current low dose CT provides additional information on disease extent, as purely lucent (cystic) FD-lesions may show no or minimal uptake on Na[18F]F-PET/CT and also causes other than FD inducing high Na18F-accumulation may be identified (osteoarthritis, fracture and osteosynthesis material). Quantitative analysis of Na[18F]F-PET/CT: normal vs pathological bone All analyses were performed independently by two nuclear medicine physicians (WvdB # 1 and DV #2), with 13 years and 10 years of experience in reading PET/ CT-CT respectively, taking both PET/CT and low-dose CT into account to describe 6