Know Your ABCS: Aneurysmal Bone Cyst of the Temporal Bone

Background: Aneurysmal bone cysts (ABCs) are benign vascular bone tumors that can grow rapidly and destroy surrounding bone. These tumors are either primary (70%) or occur secondarily to other nonmalignant bone lesions. ABCs can theoretically arise in any bone but typically develop in long bones and the posterior spinal elements. Although cranial ABCs have been reported in the literature, ABCs involving the skull base are exceedingly rare. We report a case of an ABC involving the mastoid portion of the temporal bone and highlight the key imaging features and treatment options.

Methods: Case report.

Results: A 30-year-old otherwise-healthy female presented with a 1-month history of tingling in the right cheek and tongue and progressively worsening pressure-like headaches in the right occipital region. Her headaches radiated anteriorly, worsened with dynamic head movements and Valsalva maneuvers, and were intermittently associated with right eye blurred vision. Initial CT and MR imaging revealed a multiseptated mass with fluid-fluid levels in the right lateral posterior fossa (Fig. 1). The lesion underwent surveillance imaging, and follow-up three months later revealed interval enlargement (Fig. 2). Brain MRI showed a greater degree of hyperintense T1 and T2 signal with a new central wedge-shaped area (Fig. 2A–D). Noncontrast head CT showed areas of hyperdensity consistent with blood products, and CT venogram demonstrated occlusion of the right sigmoid sinus at the level of the mass (Fig. 2E, F). Subsequent catheter angiography confirmed occlusion of the ipsilateral transverse-sigmoid sinus junction but showed no abnormal vascularity. She was taken for definitive en bloc gross total resection; intraoperatively an extradural mass was visualized emanating from the mastoid portion of the temporal bone and causing compressive occlusion, but no invasion of the ipsilateral sigmoid sinus was identified (Fig. 3A, B). Histopathology confirmed an aneurysmal bone cyst (Fig. 3C–E). FISH was negative for USP6 gene rearrangement, a marker that is observed in 70% of primary aneurysmal bone cysts but not seen in secondary cases.

Conclusion: ABCs of the skull base are rare and require nuanced review of multimodal cranial imaging for preoperative consideration. Typical MRI findings include multiple septations with fluid levels and areas of T1 and T2 hyperintensity; however, these features can also be found in other bone tumor types. Given that ABCs continue to expand until treated, surgical resection is the treatment of choice for skull base lesions that are anatomically amenable, and a gross total resection should be attempted due to high rates of recurrence.

Fig. 1 Initial MRI and CT:

Fig. 2 Interval enlargement.

Fig. 3 Pathology.