Petrous Apex Lesion
Logan A. McLean, MD & H. Ric Harnsberger, MD
DIFFERENTIAL DIAGNOSIS
Common
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Asymmetric Marrow, Petrous Apex
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Trapped Fluid, Petrous Apex
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Cholesterol Granuloma, Petrous Apex
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Primary or Metastatic Disease
Less Common
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Cephalocele, Petrous Apex
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Meningioma, T-Bone
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Fibrous Dysplasia, T-Bone
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Schwannoma, Trigeminal, Skull Base
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Cholesteatoma, Petrous Apex
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Chondrosarcoma, Skull Base
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Langerhans Histiocytosis, Skull Base
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Apical Petrositis
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Mucocele, Petrous Apex
ESSENTIAL INFORMATION
Key Differential Diagnosis Issues
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Best imaging tool
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CT and MR are complementary
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Bone CT to evaluate petrous apex (PA) “bony” expansion or destruction
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MR for lesion tissue analysis; characteristic signal per diagnosis
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Beware confusing high T1 MR signal in trapped fluid with PA cholesterol granuloma
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When protein content is high, T1 signal may be high
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In absence of bony expansion on CT, lesion should be considered trapped fluid
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Helpful Clues for Common Diagnoses
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Asymmetric Marrow, Petrous Apex
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Asymmetric pneumatization makes contralateral fatty marrow conspicuous
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CT findings
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Nonexpansile fat density PA
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MR findings
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High T1 normal fatty marrow
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Trapped Fluid, Petrous Apex
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Remote otomastoiditis leaves behind PA air cell fluid of variable protein content
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CT findings
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Opacified air cells, trabeculae present
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MR findings
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Low T1, high T2 signal
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Cholesterol Granuloma, Petrous Apex
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Chronic otitis media; pneumatized PA with recurrent hemorrhage
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CT findings
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Smooth, expansile margins
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Larger lesions affect clivus, jugular tubercle, ICA
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MR findings
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High T1 and T2 signal in expanded PA
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Primary or Metastatic Disease
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Rhabdomyosarcoma (primary or mets), neuroblastoma
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Metastasis occurs late in the disease process
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Dx of primary disease is usually known
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Radiographic appearance is variable
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Helpful Clues for Less Common Diagnoses
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Cephalocele, Petrous Apex
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Incidental PA lesion where Meckel cave appears herniated into subjacent PA
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CT findings
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Expansile ovoid lesion
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MR findings
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Low T1, high T2 “pseudopod” from Meckel cave to PA; nonenhancing
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Meningioma, T-Bone
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CT findings
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Permeative
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Sclerotic or hyperostotic bony changes
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MR findings
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Dural-based mass invades PA with avid contrast-enhancement, dural tail
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Fibrous Dysplasia, T-Bone
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Bone disorder of younger women (< 30 years) with progressive replacement of normal marrow by mixture of fibrous tissue and disorganized trabeculae
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Active phase: Cystic
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Least active/burned out: Sclerotic
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CT findings
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Expansile bone lesion with mixed ground-glass/sclerotic and cystic components sparing otic capsule
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MR findings
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Low T1 and T2 signal, foci of enhancement common
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Schwannoma, Trigeminal, Skull Base
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Cholesteatoma, Petrous Apex
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Congenital or acquired PA cholesteatoma
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CT findings
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Smooth, expansile, low-density lesion
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MR findings
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Low T1, high T2
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Nonenhancing PA lesion with restricted diffusion on DWI
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Chondrosarcoma, Skull Base
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Originates from petrooccipital fissure
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CT findings
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Characteristic chondroid matrix in 50%, invasive bony changes
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MR findings
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T2 high signal, mixed enhancement
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Langerhans Histiocytosis, Skull Base
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Langerhans histiocytes proliferation forming lytic sites in skull and skull base
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Peds
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Onset at 1 year; multifocal < 5 years
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CT findings
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Lytic lesion with beveled margins
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MR findings
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Avidly enhancing soft tissue mass
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Apical Petrositis
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Fever, retroorbital pain, diplopia, otorrhea
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CT findings
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Bony destructive changes
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MR findings
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Enhancing thick dura, PA pus does not enhance
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Mucocele, Petrous Apex
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Mimics cholesteatoma; no DWI restriction
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CT findings
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Expansile, smooth margined lesion
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MR findings
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