Foramen of Monro Mass

Karen L. Salzman, MD

DIFFERENTIAL DIAGNOSIS

Common

CSF Flow Artifact
Cavum Septi Pellucidi (CSP)
Colloid Cyst

Less Common

Neurocysticercosis
Tuberous Sclerosis Complex (Subependymal Nodule)
Subependymal Giant Cell Astrocytoma (SGCA)
Metastasis, Intraventricular
Astrocytoma (Fornix, Septum Pellucidum)
Subependymoma
Central Neurocytoma
Germinoma
Vertebrobasilar Dolichoectasia (VBD)

Rare but Important

Choroid Plexus Papilloma
Choroid Plexus Cyst
Cavernous Malformation
Ependymal Cyst
Alexander Disease

ESSENTIAL INFORMATION

Key Differential Diagnosis Issues

Foramen of Monro connects inferior lateral ventricles with 3rd ventricle
Majority of foramen of Monro lesions are related to flow artifact or a normal variant
Colloid cyst is most common, representing 15-20% of intraventricular masses
SGCA is most common in a child
Age is a helpful differentiating feature

Helpful Clues for Common Diagnoses

CSF Flow Artifact
- Multiplanar technique confirms artifact
- Look for phase artifact
Cavum Septi Pellucidi (CSP)
- Cystic CSF cavity of septum pellucidum between frontal horns, normal variant
- Often associated with a posterior continuation, cavum vergae
- Follows CSF on all sequences
- May have associated mass effect
Colloid Cyst
- Hyperdense mass at foramen of Monro on CT is characteristic
- Pillars of fornix straddle, drape around cyst
- Attached to anterior 3rd ventricular roof
- Cysts typically do not enhance, but may have “rim-enhancement” on MR

Helpful Clues for Less Common Diagnoses

Neurocysticercosis
- Cyst with “dot” inside (vesicular stage)
- Convexity subarachnoid spaces most common; ventricles least common
- Intraventricular cysts are often isolated
- Imaging varies with development stage, host response
Tuberous Sclerosis Complex (Subependymal Nodule)
- Calcified subependymal nodules 98%
- Cortical/subcortical tubers, 70-95%
- White matter lesions along lines of neuronal migration
- If subependymal nodule at foramen of Monro enlarges, likely a SGCA
Subependymal Giant Cell Astrocytoma (SGCA)
- Enhancing mass at foramen of Monro in tuberous sclerosis (TS) patients
- Occurs in 15% of TS patients
- Often cause ventricular obstruction
Metastasis, Intraventricular
- Primary tumor often known
- Often multiple lesions at gray-white junctions
- Typically involve choroid plexus if intraventricular
Astrocytoma (Fornix, Septum Pellucidum)
- Often spreads into fornix or septum pellucidum from corpus callosum
- Primary tumor involvement less common
- Imaging varies with tumor grade
Subependymoma
- T2 hyperintense, lobular, nonenhancing, intraventricular mass
- Intraventricular, inferior 4th ventricle typical (60%)
  - Lateral & 3rd ventricles less common
- Lateral ventricle: Attached to septum pellucidum or lateral wall
- May occur at foramen of Monro
- Typically no or mild enhancement
Central Neurocytoma
- “Bubbly” mass with moderate to strong enhancement
- Lateral ventricle, attached to septum pellucidum
Germinoma
- Propensity to hug midline near the 3rd ventricle ˜ 80-90%
- Pineal region: 50-65%; suprasellar: 25-35%
- Primary intraventricular germinoma is rare, typically 3rd ventricle
- Ventricles often involved by CSF seeding
Vertebrobasilar Dolichoectasia (VBD)
- Long segment irregular fusiform or ovoid arterial dilatation
- Typically occurs in vertebrobasilar system more than carotid
  - Extreme VBD can cause hyperdense foramen of Monro mass
- Look for “flow void”, phase artifact on MR
- CTA is also diagnostic for this pseudomass