Acute Enlargement of a Frontal Calvarial Lesion in Polyostotic Fibrous Dysplasia Associated with McCune-Albright Syndrome: A Case Report.

Background McCune-Albright syndrome is a rare mosaic disorder caused by postzygotic activating mutations in GNAS , classically presenting with fibrous dysplasia (FD), hyperpigmented macules, and endocrinopathies. Craniofacial FD is common and typically follows a benign course. However, acute symptomatic changes with aggressive imaging features may mimic malignant transformation or secondary lesions such as aneurysmal bone cysts (ABCs), creating significant diagnostic uncertainty. Case Description We report a 20-year-old woman with known MAS and polyostotic FD who presented with a 4-week history of progressive left frontal swelling, headache, and tenderness. Interval imaging revealed a rapidly enlarging erosive lytic lesion of the left frontal calvarium with peripheral enhancement and an internal fluid-fluid level, raising concern for secondary pathology or malignancy. Given rapid growth and cortical erosion, the patient underwent surgical resection with cranioplasty. Intraoperatively, the lesion contained serosanguinous fluid. Histopathology demonstrated bland spindle cell proliferation with curvilinear woven bone trabeculae and hemosiderin-laden macrophages, without cytologic atypia, necrosis, or features of ABC. Molecular testing confirmed a GNAS p.(R201C) mutation, consistent with FD. The patient had an uncomplicated postoperative course with complete symptom resolution. Conclusion This case highlights that fluid-fluid levels, rapid interval growth, and erosive changes can occur in craniofacial FD without ABC formation or malignant transformation. Imaging findings alone are insufficient to distinguish benign changes from aggressive pathology. Accurate diagnosis requires a multimodal approach integrating clinical context, advanced imaging, histopathology, and pathologic analysis. Surgical intervention is justified in some cases to establish diagnosis and provide symptomatic relief.