Central Neurocytoma - Imaging Features

Overview

Location (Key Imaging Clue)

• Intraventricular: almost exclusively within the lateral ventricles, anteriorly near the foramen of Monro
• Arises from or is closely related to the septum pellucidum and columns of the fornix
• Can extend into the third ventricle through the foramen of Monro
• Rarely, extraventricular neurocytomas arise in cerebral hemispheres, cerebellum, or spinal cord

CT Features

• Well-demarcated, lobulated intraventricular mass
• Calcification is common (frequently visible on CT)
• Small cysts are frequently present
• May be iso- to slightly hyperdense relative to brain parenchyma
• Obstructive hydrocephalus is a common associated finding due to CSF obstruction at the foramen of Monro

MRI Features

Characteristic Morphology on MRI

• "Bubbly" or "soap-bubble" appearance: multiple septated cysts within the mass
• Heterogeneous signal due to mixture of solid nodules (isointense to gray matter), cysts, calcifications, and flow voids
• Gray-matter-isointense nodules interspersed within the cystic components
• Sharply demarcated margins, non-infiltrative
• No significant surrounding edema (unlike high-grade tumors)

Advanced MRI / Functional Imaging

• MR Spectroscopy: Elevated glycine peak at 3.55 ppm (relatively specific for central neurocytoma); elevated choline, reduced NAA
• DWI: Variable; usually not restricted diffusion (unlike high-grade tumors)
• Perfusion MRI: May show elevated rCBV in solid components

Key Differential Diagnoses

Clinical Correlation

• Contrast-enhanced MRI is the modality of choice
• The diagnosis should be considered in any young adult presenting with an anterior intraventricular mass near the foramen of Monro showing cysts, calcification, and heterogeneous enhancement
• Key histological confirmation: synaptophysin-positive immunostaining distinguishes it from oligodendroglioma (which lacks this marker)
• Atypical neurocytomas (MIB-1/Ki-67 >2-3%) have higher recurrence rates

Cerebellopontine (CP) Angle Mass Lesion - Differential Diagnosis

Boundaries of the CP Angle

• Anterior: CN VI and lateral clivus
• Lateral: medial surface of the petrous bone
• Medial: pons, middle cerebellar peduncle, ventral cerebellum
• Superior: CN V (trigeminal)
• Inferior: CN IX, X, XI
• Contained within the space: CN V, VII, VIII, IX, X, XI and the AICA

Relative Frequency

Common Lesions

1. Vestibular Schwannoma (Acoustic Neuroma) - MOST COMMON

• Benign schwannoma of the vestibular division of CN VIII
• Arises within the internal auditory canal (IAC), expands into the CPA
• Unilateral in 95%; bilateral = NF2 (chromosome 22 mutation)
• CT: Enlargement of IAC (>2 mm asymmetry), no calcification
• MRI: T1 isointense, T2 slightly hyperintense, enhances with gadolinium (moderate-intense, homogeneous or heterogeneous for large lesions)
• Characteristic "ice cream cone" shape with IAC component + CPA component
• No dural tail

Axial T1-weighted MRI: left isointense CPA vestibular schwannoma (arrow). Hyperintense on T2, enhances with contrast. - Cummings Otolaryngology

2. Meningioma (~10%)

• Second most common CPA lesion
• Eccentric to the IAC (broad dural base; does NOT enter the IAC)
• CT: Iso- to hyperdense; may show calcification; adjacent hyperostosis
• MRI: T1 isointense; T2 signal varies with calcium content (hypo- or hyperintense); intensely and homogeneously enhances
• Dural "tail" sign - enhancing dural extension pathognomonic
• IAC involvement absent or shallow; no IAC erosion/widening typical of schwannoma

3. Epidermoid Cyst (Primary Cholesteatoma)

• Derived from ectodermal remnants; filled with keratin debris
• Has a characteristic "insinuating" growth pattern - wraps around structures rather than displacing them
• CT: Hypodense, no enhancement
• MRI: T1 hypointense, T2 hyperintense (similar to CSF at first glance)
• Key distinguishing feature: DWI - restricted diffusion (bright signal), unlike arachnoid cyst which follows CSF
• No enhancement with gadolinium
• FLAIR: higher signal than CSF (unlike arachnoid cyst)
• MR spectroscopy: elevated aniline, glutamine/glutamate, choline; absent/reduced NAA and creatinine

4. Arachnoid Cyst

• Follows CSF signal on ALL sequences (T1 hypo, T2 hyper, FLAIR suppressed like CSF)
• No enhancement, No DWI restriction - key distinction from epidermoid
• Smooth, well-defined, displaces structures (does not insinuate)

5. Facial Nerve Schwannoma (~1%)

• Imaging nearly identical to vestibular schwannoma
• Clue: may involve the geniculate ganglion or extend along the facial nerve canal
• Presents with facial palsy (unusual for acoustic schwannoma except in late/large disease)

Less Common Lesions

MRI Imaging Characteristics Summary Table

Clinical Approach

1. Does it involve/expand the IAC? - Yes → schwannoma
2. Is there a dural tail? - Yes → meningioma
3. Does it restrict on DWI? - Yes → epidermoid
4. Does it follow CSF on all sequences? - Yes → arachnoid cyst
5. Is it T1 bright? - Fat = lipoma; cholesterol crystal + blood = cholesterol granuloma