Endocrine Abstracts

Introduction: Fibrous dysplasia (FD) is a rare mosaic disorder originating from the activating mutation in the gene encoding the alpha subunit of the Gs stimulatory protein (GNAS) in multipotent skeletal stem cells that cannot differentiate into bone-forming osteoblasts. FD is thought to be a self-limited disease with a peak of activity in childhood and a waning of activity in adulthood.

Case presentation: We report a 39-year-old woman with mild polyostotic FD and precocious puberty in her history. At the age of 31, the patient underwent total thyroidectomy for papillary thyroid cancer, complicated by postoperative permanent hypoparathyroidism. The ¹⁸F- NaF PET/CT, carried out to monitor skeletal activity, showed several new skeletal lesions that were not documented on the original PET/CT with ¹⁸F-fluorodeoxyglucose performed 19 years ago when FD was diagnosed. Four new lesions were localized in the ribs, two in the vertebrae and one in the symphysis, with corresponding changes in the CT image characteristic of FD. The patient had no clinical evidence of fractures and the thyroid cancer was in remission at the time of the scan. In contrast to polyostotic FD, osteocalcin, alkaline phosphatase and collagen beta crosslaps, which reflect bone remodeling and should correlate with disease extent, were within normal ranges in our patient. The only exception was procollagen 1 N-terminal propeptide (P1NP), which was consistently 3–4 times higher (mean value of P1NP in years 2021–2023: 219.2 ng/ml, normal values 15.3–58.8 ng/ml).

Conclusion: The present case documents the evolution of new skeletal lesions on ¹⁸F-NaF PET/CT imaging in an adult patient with mild FD/ McCune-Albright syndrome and shows that the skeletal burden in FD may not be completely stable in adulthood. Furthermore, concomitant postoperative hypoparathyroidism modulates the patient’s bone turnover and may highlight a possible role of parathyroid hormone in skeletal involvement in FD. Supported by MH CZ - DRO (Institute of Endocrinology – EÚ, 00023761).