A rare calvarial aneurysmal bone cyst in an adult: Case report and literature review.

Background: Aneurysmal bone cysts (ABCs) are uncommon benign osteolytic lesions that typically occur in children and adolescents and rarely involve the calvarium. In adults, ABCs are more often secondary lesions that arise within pre-existing bone pathology, the most common being fibrous dysplasia (FD). Case Description: We report the case of a 35-year-old woman who presented with progressive pulsatile pain over the left parietal scalp and a newly palpable mass. Imaging demonstrated extensive FD of the temporal bone and sphenoid wing with a large, multiloculated expansile lesion exhibiting fluid-fluid levels, findings consistent with a secondary ABC. Given the worsening symptoms, size of the lesion, and cortical thinning, the patient underwent image-guidance-assisted resection. A staged synthetic cranioplasty was subsequently performed. Histopathologic examination confirmed FD with superimposed ABC, without malignant features. The patient recovered with excellent neurological and cosmetic outcomes. Conclusion: Secondary ABCs arising within the FD of the calvarium are exceedingly rare in adults. This case reinforces the importance of recognizing this entity in the differential diagnosis of expansile calvarial lesions. Complete surgical excision with margin control and staged cranioplasty provides definitive treatment, minimizes recurrence risk, and restores cranial integrity.