Giant Posterior Temporal Bone Arachnoid Granulations: CT and MRI Findings

Objective: The radiologic appearance of arachnoid granulations (AGs) in typical locations is well established and they are rarely mistaken for other pathologies. However, when large and seen in atypical locations, such as along the posterior petrous bone, AGs can be a source of diagnostic and therapeutic confusion. We present two cases of giant posterior temporal bone AGs and review their clinical presentation, potential complications, and an imaging-based differential diagnosis. Patients: Two patients with surgically or pathologically proven giant symptomatic AGs in the posterior petrous bone. Main Outcome Measure: Clinical presentation, radiological features, surgical findings, and potential complications of giant AGs. Results: In two middle-aged women (37 and 55 years), computed tomography (CT) demonstrated solitary large lytic lesions in the posterior right petrous temporal bone. These were similar in appearance to cerebrospinal fluid (CSF) on magnetic resonance imaging (MRI), though they exhibited some minor deviations such as thin internal septations, mild peripheral enhancement, and heterogeneous signal on fluid-attenuated inversion recovery (FLAIR). The MRI appearance effectively distinguished the giant AGs from other lesions that can occur in this area such as endolymphatic sac tumor (ELST). Surgery was successfully performed to prevent complications from a CSF leak. Conclusion: The posterior temporal bone is an atypical location for AGs and can lead to diagnostic confusion, particularly when they are large. Familiarity with the characteristic imaging appearance of giant AGs in this location can help avoid misinterpretation as a more aggressive pathology and help recognize patients who are at risk for a CSF leak.