Clival Aneurysmal Bone Cyst With Cavernous Sinus Extension: A Diagnostic Challenge.

Aneurysmal bone cyst (ABC) is a benign but locally aggressive osseous neoplasm that predominantly affects children and young adults, with skull base involvement being uncommon. We report a 59-year-old woman with a progressively enlarging clival ABC extending into the sphenoid sinus and right cavernous sinus. She initially presented with right frontal headache and subsequently developed progressive diplopia, ptosis, ophthalmoplegia, and trigeminal sensory impairment. An initial endoscopic endonasal transsphenoidal biopsy was interpreted as a mucocele; however, continued clinical deterioration and radiological progression prompted repeat tissue sampling through a transcranial microsurgical approach. Histopathological and immunohistochemical findings, together with the detection of a USP6 gene rearrangement by fluorescence in situ hybridization, supported the diagnosis of primary ABC in the appropriate clinicopathological context. Complete surgical resection was not feasible because of extensive skull base and cavernous sinus involvement. The patient was referred to clinical oncology for consideration of denosumab; however, treatment was not recommended because the available supporting evidence was considered insufficient. The lesion continued to progress, with subsequent neurological deterioration, and the patient died approximately 10 months after the definitive diagnosis. This case illustrates the diagnostic challenges of ABC in an unusual anatomical location and age group and emphasizes the importance of clinicoradiological-pathological correlation, diagnostic reassessment when findings are discordant, and the appropriate use of ancillary molecular testing in selected cases.