Association between serum FGF23 and [18 F]FDG PET/CT uptake metrics in fibrous dysplasia

Abstract Purpose To investigate the relationship between serum fibroblast growth factor-23 (FGF23) and [^18F]FDG PET/CT metabolic parameters in fibrous dysplasia (FD), and to describe the performance of FDG PET/CT across the FD/McCune–Albright spectrum, including low-uptake lesions. Methods In this single-centre retrospective cohort study, consecutive adult patients with FD who underwent [^18F]FDG PET/CT and had an available serum FGF23 measurement within the same clinical work-up were included. The index lesion (highest uptake) was analysed per patient. Semi-automated VOIs were generated on a GE Advantage workstation using a fixed percentage-of-SUVmax isocontour. SUVmax and SUVmean were recorded for the index lesion; contralateral bone, blood pool, and liver SUVmax were also collected. Lesion-to-blood-pool and lesion-to-liver SUVmax ratios were calculated as reproducible reference-normalized uptake indices. MTV and TLG were computed when VOI delineation was feasible. Associations between FGF23 and PET metrics were assessed using Pearson correlation (pairwise complete observations), with additional sensitivity analyses for skewed distributions. Results Forty patients were included (female 65.0%); clinical subtypes were monostotic FD (32.5%), polyostotic FD (55%), and McCune-Albright syndrome (12.5%). Serum FGF23 was available in 40/40 patients (49.58 ± 19.66 pg/mL). Index-lesion uptake was heterogeneous (osseous SUVmax 5.71 ± 4.34; SUVmean 3.44 ± 2.52). FGF23 correlated positively with osseous SUVmax ( r  = 0.396; p  = 0.011; n  = 40) and osseous SUVmean ( r  = 0.400; p  = 0.011; n  = 40); after Benjamini-Hochberg false discovery rate (BH-FDR) correction within the prespecified Pearson correlation family, both associations remained significant (q = 0.040 for each). Lesion-to-blood-pool and lesion-to-liver SUVmax ratios showed positive but non-significant trends. MTV/TLG were not significantly associated with FGF23 (both n  = 31). MTV/TLG were not computable in 9 patients because low/mild lesion uptake near background prevented reliable threshold-based segmentation. Conclusion In FD, higher serum FGF23 is associated with greater osseous FDG uptake intensity (SUVmax/SUVmean), supporting a link between systemic phosphate-regulatory biology and PET-derived metabolic activity. However, a clinically relevant subset of low-uptake lesions limits volumetric quantification (MTV/TLG) and highlights that FDG PET/CT may under-represent low-activity disease when threshold-based segmentation is required.