Description
Infratentorial ependymomas are a type of glial tumour that originates from ependymal cells lining the ventricular system in the posterior fossa, often involving the fourth ventricle. They are part of the spectrum of intracranial ependymomas, distinguished by their location in the infratentorial compartment. Ependymomas most commonly present as infratentorial tumours.
Pathogenesis
Ependymomas are believed to originate from radial glial cells or their progenitors which are cells that function both as neural progenitors and as a scaffold for neuronal migration during development. These tumours often demonstrate a propensity to spread along CSF pathways due to their origin near or within the ventricular system.
Subtype
Histological
Histological grading refers to the appearance of the cells under a microscope and how quickly they are likely to grow and spread.
- Classic (WHO Grade II): Characterised by well-circumscribed, often cystic, tumours with variable cellularity, perivascular pseudorosettes, and ependymal rosettes. This is the most common subtype.
- Anaplastic (WHO Grade III): Features include high cellularity, nuclear atypia, mitotic activity, and regions of necrosis.
Molecular
Molecular subtyping of ependymomas in the posterior fossa (EPN-PF) looks at the genetic changes in the tumour cell which can help predict the tumour’s behaviour, its likely response to treatment, and the patient’s prognosis.
- Group A (EPN-PFA): Predominantly infratentorial, mainly observed in infants and young children. It is associated with poor prognosis.
- Group B (EPN-PFB): Predominantly infratentorial, mostly seen in older children and adults. It is associated with a better prognosis compared to Group A.
Epidemiology, Risk Factors & Associations
- Most common in children, especially between 5-10 years of age.
- No clear sex predilection.
- Associated with multiple intracranial schwannomas, meningiomas, and ependymomas (MISME) syndrome, a variant of Neurofibromatosis Type 2 (NF2) (5-15% of cases).
Clinical Features
- Symptoms related to increased intracranial pressure due to obstruction of CSF flow, such as headache, nausea, vomiting, and gait disturbance.
- Cerebellar signs, including ataxia, nystagmus and dysmetria.
Complications
- Hydrocephalus due to obstruction of the fourth ventricle.
- Metastatic disease along CSF pathways.
- Neurologic deficits due to mass effect or involvement of neural structures.
Pathological Features
Histopathology
- Macroscopic: Often well-circumscribed, grey-pink tumours which may be solid, cystic or both.
- Microscopic: Characteristically demonstrate perivascular pseudorosettes and ependymal rosettes. Anaplastic variant demonstrates increased cellularity and mitotic activity.
Serology
No specific serologic markers.
Biochemistry
No specific biochemical markers.
Radiological Features
General Features
- Typically seen as well-defined, lobulated, homogeneous mass in the posterior fossa, most often arising from the floor fourth ventricle.
- Described as a soft “plastic” or “toothpaste” tumour, with a propensity to squeeze out through the 4th ventricular foramina into basal cisterns.
- Anterolateral extension through foramina of Luschka into cerebellopontine angle cistern
- Posteroinferior extension through the foramen of Magendie into cisterna magna
- Can demonstrate cystic components and fine, stippled calcifications.
- Generally enhances uniformly after contrast administration.
- Associated obstructive hydrocephalus is common.
- MISME: In patients with this variant of NF2, may see bilateral vestibular schwannomas, multiple meningiomas, and, less commonly, spinal ependymomas.
CT
- Non-contrast: Iso- to hyperdense compared to brain parenchyma. Calcifications seen in approximately 30% of cases.
- Contrast-enhanced: Shows uniform enhancement.
MRI
- T1WI: Iso- to hypointense relative to brain. Heterogeneity and hyperintense foci may represent calcification and haemorrhage.
- T2WI: Hyperintense cystic foci, hypointense calcification.
- FLAIR: Hyperintense cystic components
- SWI/GRE: Hypointense blooming artefact representing calcification and haemorrhage
- T1 C+: Uniform or heterogenous enhancement.
- DWI/ADC: No significant restricted diffusion unless associated with cystic components.
NM
- PET: Increased FDG uptake. May help differentiate recurrent tumour from radiation necrosis.
Grading and Staging
The World Health Organisation (WHO) grading system is used, classifying ependymomas into:
- Grade I (myxopapillary ependymoma and subependymoma)
- Grade II (classic ependymoma)
- Grade III (anaplastic ependymoma).
Diagnosis
The diagnosis is based on histopathological examination of the tumour tissue obtained during surgery, supplemented by radiological imaging.
Differential Diagnosis
- Medulloblastoma: Typically hyperdense on non-enhanced CT. Characteristically demonstrates diffusion restriction (increased DWI, decreased ADC). Most commonly arises from the roof of the 4th ventricle, with 90% found in the midline. Compared to infratentorial ependymomas, these often have a more distinct interface with the floor of the 4th ventricle.
- Cerebellar Pilocytic Astrocytoma: Originates from cerebellar parenchyma. Unlike ependymomas, approximately 90% are off midline. Cystic component is present in most cases. The enhancing portion shows vigorous enhancement in over 90% of cases, which may help differentiate from ependymomas.
- Choroid Plexus Papilloma: Intraventricular tumour that exhibits vigorous enhancement. Although ependymomas are typically found in the 4th ventricle in children, in adults, a choroid plexus papilloma is more common in this location. A lobulated, frond-like border will help differentiate from ependymomas.
- Atypical Teratoid/Rhabdoid Tumour: Cellular mass that demonstrates diffusion restriction (increased DWI, decreased ADC). More likely to be off midline compared to medulloblastoma (MB). Compared to MB, more likely to have cysts and haemorrhage. Typically occurs in very young children, which can help differentiate from ependymomas, which are more common in older children and adults.
- Brainstem Glioma: Infiltrative mass that expands the brainstem. Demonstrates fairly homogeneous signal on MRI. May project dorsally into the 4th ventricle, which can help differentiate from infratentorial ependymomas which tend to originate in the 4th ventricle.
Management
- Surgical resection is the mainstay of treatment, with an attempt at gross total resection whenever feasible.
- Adjuvant radiotherapy is often used, particularly in cases of incomplete resection or higher-grade tumours.
- Will require completing imaging of neuroaxis to complete staging.