In a 62‑year‑old woman with an isodense expansile lesion involving the petrous apex of the sphenoid bone and contiguous clivus on non‑contrast computed tomography, what is the most likely diagnosis and what is the appropriate next diagnostic step?

Most Likely Diagnosis: Cholesterol Granuloma

The most likely diagnosis is a cholesterol granuloma of the petrous apex, and the appropriate next diagnostic step is MRI with and without contrast to confirm the diagnosis before considering any biopsy.

Diagnostic Reasoning

The clinical presentation of an isodense, expansile lesion at the petrous apex extending to the clivus on non-contrast CT is highly characteristic of cholesterol granuloma 1. This is the most common expansile lesion of the petrous apex and has pathognomonic imaging features that can establish the diagnosis without biopsy in most cases.

Key Imaging Characteristics

The isodense appearance on CT is a classic finding for cholesterol granuloma 1. The lesion typically presents as:

• A smooth-walled, expansile, non-enhancing mass
• Isodense with brain tissue on CT
• Often associated with well-pneumatized mastoids and petrous apices 1

Critical Next Step: MRI

MRI is essential and diagnostic for cholesterol granuloma 2. The characteristic MRI findings that will confirm the diagnosis are:

• High signal intensity on BOTH T1-weighted and T2-weighted images (this is pathognomonic) 2
• This "bright on both sequences" pattern reliably distinguishes cholesterol granuloma from other petrous apex lesions 2

Differential Considerations

While cholesterol granuloma is most likely, the differential for expansile petrous apex lesions includes 2, 3:

• Cholesteatoma: Would show low T1 signal (unlike cholesterol granuloma)
• Arachnoid cyst: Would follow CSF signal on all sequences
• Chondrosarcoma: More aggressive bone destruction, different age presentation
• Plasmacytoma: Rare at this location, would show enhancement 4

Why MRI Before Biopsy

Do not proceed directly to biopsy 3. The imaging features on MRI are so characteristic for cholesterol granuloma that biopsy is typically unnecessary and potentially harmful given the location 2, 3. MRI can reliably distinguish between the main differential diagnoses, which is critical because:

• Cholesterol granulomas require only drainage and stenting 1
• Cholesteatomas require aggressive surgical removal, often sacrificing hearing 3
• Misdiagnosis leads to inappropriate treatment 3

Clinical Correlation

Look for symptoms of cranial nerve involvement:

• Trigeminal nerve dysfunction (CN V)
• Facial nerve palsy (CN VII)
• Abducens nerve palsy (CN VI)
• Notably, hearing is often preserved initially 1

Management Algorithm

1. Obtain MRI with and without IV contrast immediately 5, 2
2. If MRI shows high T1 and T2 signal → Diagnosis confirmed as cholesterol granuloma
3. If confirmed, treatment is surgical drainage with stent placement for permanent aeration 1
4. Hearing preservation is achievable in most cases (83% in one series) 1

Common Pitfall

The major pitfall is proceeding to aggressive surgical resection without proper MRI characterization, potentially sacrificing hearing unnecessarily when simple drainage would suffice 3. The "typical" imaging features are diagnostic in most but not all cases, so tissue diagnosis may ultimately be needed if imaging is atypical 3.