199 Denosumab reduces lesional Fluoride skeletal burden on Na[18F]F PET-CT Discussion In this study we show that Na[18F]F PET-CT is capable of detecting therapy-induced changes in lesional bone turnover (especially FTV and FAS) in patients treated with the RANKL-inhibitor denosumab. This underwrites previous findings of induced normalization of markers of bone turnover after initiation of denosumab in patients previously treated with bisphosphonates as this not only reflects a biochemical systemic response but also a local response where this could not be observed in the past with bisphosphonates [2]. Therefore Na[18F]F PET-CT can be used as an objective parameter of local treatment response in FD/MAS. Also, we did not observe any change in the uptake of healthy bone meaning that the decrease in activity is only lesional, not systemically. Our data also showed that Na[18F]F PET-CT observed changes in FD-burden correlated with observed decreases in bone turnover measured by BTMs reflecting that the observed changes in BTMs do reflect a reduction in local FD burden activity. In clinical practice, change of biochemical bone remodeling biomarkers together with change in FD-related skeletal pain are known outcome measures [2]. An advantage of imaging over a blood test is the visualization of localization of either response or progress, as not all localizations necessarily might respond equally to therapy. In 5 out of 15 patients, serum BTMs failed to capture response of FD-burden to therapy although clinically improvement was expected, especially in patients with relatively small baseline FTV, and thus low skeletal burden. In contrast, Na[18F]F PET-CT did reveal decrease in local FD-activity. Although bone scan with the SBS representing affected bone segments is widely used as an adjunct for primary characterization of FD, it might be an underestimation for FD-related disease burden to our and other’s experience [4]. Moreover, the use of SBS in follow up during treatment with bisphosphonates (including pamidronate) remains controversial [2, 4, 5]. A single case report showed in different disease, a giant cell tumor of bone, a significant decrease in Na[18F]F-skeletal burden after initiation of denosumab [15]. Use of the SBS in FD/MAS patients treated with denosumab, has not yet been reported. Specifically in children receiving very high doses of RANKL-inhibitors concern has been raised that osteopetrosis has been reported [16]. We have seen no signs of osteopetrosis in our population in our CT-part of the total body Na[18F]F PET-CT scans, consisting of only adults andwith dosages of denosumab as described in themethods section. These dosages might be considered a moderately increased over common dosages, but not as very high. Radiation burden for Na[18F]F PET-CT is low especially when using only 1 MBq/kg of activity (effective dose: 0.024 mSv/MBq), resulting in an 8
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