CPA Mass, Adult

H. Ric Harnsberger, MD

DIFFERENTIAL DIAGNOSIS

Common

Vestibular Schwannoma

Less Common

Meningioma, CPA-IAC
Epidermoid Cyst, CPA-IAC
Aneurysm, CPA-IAC
Arachnoid Cyst, CPA-IAC
Metastases, CPA-IAC

Rare but Important

Neurofibromatosis 2, CPA-IAC
Sarcoidosis, CPA-IAC
Choroid Plexus Papilloma, CPA
Lipoma, CPA-IAC
Ependymoma, CPA
Pseudotumor, Intracranial
Schwannoma, Facial Nerve, CPA-IAC
Schwannoma, Jugular Foramen
Hemangioma, IAC
Neurenteric Cyst

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Idealized imaging protocol in evaluating CPA mass lesions
- T1 C+ fat-saturated MR is gold standard
  - Fat-saturation differentiates lipoma from vestibular schwannoma
  - Add DWI for possible epidermoid
  - Add GRE for aneurysm wall clot & calcification; tumor calcifications
- T2 thin-section, high-resolution, MR gives more surgical data when vestibular schwannoma diagnosed
  - Amount of CSF cap in lateral IAC
  - Assessment of relationship to cochlear nerve canal
  - If small schwannoma, nerve of origin
Knowledge of relative incidence of lesions key in cerebellopontine angle
- Vestibular schwannoma ˜ 90% all CPA-IAC masses
- Meningioma, epidermoid cyst, aneurysm, arachnoid cyst together represent ˜ 8% all CPA-IAC masses
- All other diagnoses in differential list ˜ 2% of CPA-IAC masses

Helpful Clues for Common Diagnoses

Vestibular Schwannoma
- Morphology: Ovoid intracanalicular mass (IAC); “Ice cream on cone” shape (CPA-IAC)
- T1 C+ MR: Enhancing ± intramural cysts

Helpful Clues for Less Common Diagnoses

Meningioma, CPA-IAC
- Morphology: “Mushroom” dural-based mass capping IAC asymmetrically
- T1 C+ MR: Enhancing ± dural “tails” ± CSF-vascular cleft if CPA component is larger
  - 25% of CPA meningiomas have extension/dural tail into IAC
Epidermoid Cyst, CPA-IAC
- Morphology: Insinuating ± scalloping brainstem margin
- T1 C+ MR: Nonenhancing; may be difficult to see
- DWI: Restricted diffusion (high signal) makes diagnosis
Aneurysm, CPA-IAC
- Morphology: Ovoid or fusiform; rarely IAC
- T1 & T1 C+ MR: Complex signal mass from wall calcification, clot & flow
- MRA, CTA, or angiography sort out diagnosis
Arachnoid Cyst, CPA-IAC
- Morphology: Fills cistern with rounded margins
- Imaging
  - T1 C+ MR: No enhancement
  - FLAIR attenuates
  - DWI: No restricted diffusion
Metastases, CPA-IAC
- Morphology: Irregular, invasive margins
- T1 C+ MR: Single or multiple enhancing masses in CPA area
  - 4 sites primarily involved: Flocculus, choroid plexus, arachnoid-dura, or pia

Helpful Clues for Rare Diagnoses

Neurofibromatosis 2, CPA-IAC
- Morphology: Bilateral ovoid IAC or “ice cream on cone” CPA-IAC masses
- T1 C+ MR
  - Bilateral enhancing CPA-IAC masses pathognomonic of NF2
  - Other schwannomas & meningiomas may be present
Sarcoidosis, CPA-IAC
- Laboratory: CSF lymphocytosis; ↑ ↑ blood angiotensin converting enzyme (ACE)
- Morphology: En plaque or nodular dural lesion(s)
- T1 C+ MR: Enhancing multifocal dural-based lesions
Choroid Plexus Papilloma, CPA
- Morphology: Dumbbell shape with 4th ventricle and CPA cistern components
  - Pear-shaped if begins in foramen of Luschka
- T1 C+ MR: Avidly enhancing mass in 4th ventricle projecting through foramen of Luschka into CPA cistern
Lipoma, CPA-IAC
- Morphology: Ovoid if IAC; CPA lesion may be broad-based against brainstem
- CT: Fat-density lesion of CPA ± IAC ± inner ear
- T1 MR: High signal lesion disappears with fat-saturation
- Caveat: If T1 C+ without fat-saturation, may be mistaken for vestibular schwannoma
Ependymoma, CPA
- Morphology: Irregular soft tumor squeezes out through 4th ventricle foramen of Luschka into CPA cistern
  - Tumor margins amorphous
- CT: Calcifications in 50%
- T1 C+ MR: Heterogeneous enhancement of solid tumor components
  - Marginal enhancement of tumor cyst wall
Pseudotumor, Intracranial
- Morphology: En plaque
- T1 C+ MR: Thickened, enhancing dura
- Caveat: May mimic meningioma, sarcoidosis or metastatic disease
Schwannoma, Facial Nerve, CPA-IAC
- Morphology: CPA-IAC mass with “labyrinthine tail”
- CT: Labyrinthine segment CN7 may be enlarged
- T1 C+ MR: Enhancing tubular mass in CPA-IAC and labyrinthine segment CN7
- Caveat: If not labyrinthine segment CN7 involvement, cannot differentiate from vestibular schwannoma
Schwannoma, Jugular Foramen
- T1 C+ MR: Enhancing mass arising from jugular foramen
  - Mass projects cephalad into CPA cistern
Hemangioma, IAC
- Morphology: Ovoid IAC mass with punctate calcifications
- CT: Punctate calcifications in IAC mass
- T1 C+ MR: Enhancing IAC mass with focal low signal foci (calcifications)
Neurenteric Cyst
- Morphology: Rounded ovoid mass in prepontine cistern
- MR: Intermediate to high signal T1 prepontine mass
- Caveat: T1 increased signal differentiates from epidermoid cyst