New Skeletal Lesions and Hypoparathyroidism-Modified Bone Turnover in an Adult with Fibrous Dysplasia/McCune-Albright Syndrome

Background Fibrous dysplasia (FD) is a rare, mosaic skeletal disorder characterized by replacement of normal bone with fibro-osseous tissue. Disease activity peaks in childhood and typically diminishes by adulthood. FD is associated with accelerated bone turnover, the degree of which generally reflects the extent and severity of skeletal involvement. Case presentation We report the case of a 39-year-old woman with mild polyostotic FD and precocious puberty. At the age of 31 years, the patient underwent total thyroidectomy for papillary thyroid cancer complicated by permanent hypoparathyroidism. The ¹⁸ F- Sodium Fluoride PET/CT (NaF) revealed new rib lesions not documented on the original ¹⁸ F-fluorodeoxyglucose PET/CT performed 19 years ago when FD was diagnosed. All new skeletal foci had clear morphological correlates of FD on the CT images of ¹⁸ F-NaF PET/CT. The patient had no fractures, and the thyroid cancer was in remission. In contrast to polyostotic FD, osteocalcin and alkaline phosphatase, which should correlate with disease extent, were within normal ranges. The only exception was the procollagen 1 N-terminal propeptide, which was consistently elevated 3-4 times. Next-generation sequencing, following target enrichment, identified the causal variant Arg201Cys in the GNAS gene in the DNA extracted from the dysplastic right pelvic bone. The diagnosis was changed to FD/McCune–Albright syndrome (FD/MAS) because the patient had precocious menarche at the age of 8. Conclusions The present case documents the evolution of new skeletal lesions on ¹⁸ F-NaF PET/CT in an adult patient with mild FD/MAS despite the expected quiescence of the disease. Furthermore, concomitant postoperative hypoparathyroidism modulates the patient´s bone turnover and may unmask specific dysregulation of osteoblasts in FD.