Posterior fossa ependymoma

Last revised by Wojciech Micał on 16 Aug 2022

Posterior fossa ependymomas are the most common type of ependymoma, most commonly occurring in children. They are divided on the basis of DNA-methylation profiling into two groups, A and B. 

Posterior fossa ependymomas are encountered in all age groups but are usually encountered in children. The epidemiology, however, depends on molecular characteristics 1,4

  • posterior fossa group A (PFA) ependymomas
    • primarily in infants and young children
      • <6 years of age: >95% of all posterior fossa ependymomas
      • adolescents: 50%
      • adults: 5-10%
    • somewhat more common in males
  • posterior fossa group B (PFB) ependymomas
    • primarily in adolescents and adults
      • <5 years of age: <5% of all posterior fossa ependymomas
      • adolescents: 20-50%
      • adults: 90%
    • somewhat more common in females

Initial presentation with signs and symptoms of raised intracranial pressure is common. Other posterior fossa symptoms including ataxia are also encountered 3. Supratentorial ependymomas may also present with seizures or focal neurological deficits 2,5.

In the infrequent scenario of hemorrhage, the presentation will be hyper-acute. 

Ependymomas are glial tumors with ependymal differentiation which tend to arise within or abutting the ventricular system of the brain or central canal of the spinal cord 1-5. Although for many years ependymomas were believed to be tumors arising from dedifferentiated ependymocytes, it now appears certain that they arise from an undifferentiated glial stem or progenitor cell 1,6

Macroscopically, ependymomas tend to be well defined lobulated grey or tan-colored soft and frond-like tumors which are moderately cellular. They may have focal areas of calcification. 

Microscopically, these tumors are characterized by well-differentiated cells. Characteristic features include ependymal rosettes, which are uncommon but pathognomonic and perivascular pseudorosettes which are far more common and seen in most ependymomas 1,3.

Dystrophic calcification, hemorrhage, myxoid degeneration and even rarely metaplasia (bone or cartilage) are sometimes encountered 1

A number of histological patterns are historically recognized, including papillary ependymoma, clear cell ependymoma, tanycytic ependymoma (usually found in the spinal cord). These are, however, no longer recognized as distinct subtypes 1

A number of immunohistochemical markers are useful, including 1:

Ependymoma grading is controversial and probably not as important as molecular features 18. Nonetheless, tumors can be given a grade of 2 or 3 based on histological features 18. The term "anaplastic ependymoma" is, however, no longer listed, although remains equivalent to grade 3. 

The majority of posterior fossa ependymomas arise from the lateral recess of the fourth ventricle (molecular subgroup: PFA) or midline floor of the fourth ventricle near the obex (molecular subgroup: PFB) 3-5,8.

Posterior fossa ependymomas are apt to extend through the foramina of Luschka and Magendie, hence the term "plastic ependymoma" or comparsion to a toothpaste. This is a characteristic feature and can be seen on both CT and MRI. 

Ependymomas are typically heterogeneous masses with areas of necrosis, calcification, cystic change and hemorrhage frequently seen. This results in a heterogeneous appearance on all modalities. 

Although intraparenchymal lesions are usually supratentorial, rarely they are found in the cerebellum 9, and generally large and variable in appearance, ranging from completely solid, enhancing masses to cysts with a mural nodule, or more heterogeneous masses 3. They are believed to arise from the trapping of embryonic rests of ependymal tissue in the developing brain parenchyma 5.

  • coarse calcification is common (50%) 
  • cystic areas (50%) 
  • solid component iso- to hypodense
  • heterogeneous enhancement 
  • variable hemorrhage
  • T1
    • solid portions of ependymoma typically are isointense to hypointense relative to white matter 3
  • T2
    • hyperintense to white matter
    • more reliable in differentiating tumor margins than non-contrast T1-weighted images (but less reliable than contrast-enhanced T1)
  • T2* (e.g. SWI)
    • foci of blooming from hemorrhage or calcification
  • T1 C+ (Gd)
    • enhancement present but heterogeneous
    • enhancement with gadolinium is useful in differentiating tumors from adjacent vasogenic edema and normal brain parenchyma
  • DWI/ADC
    • restricted diffusion may be seen in solid components, especially in anaplastic tumor
    • diffusion should be interpreted with caution in masses with significant hemorrhage or calcification
  • MRS

Although it is uncommon when compared to tumors like medulloblastomas, careful examination of the entire neuraxis is required to assess for the presence of CSF seeding

A total or partial resection may be attempted +/- irradiation.

Prognosis is, however, relatively poor, which is mainly due to tumors occurring in surgically-challenging locations, making complete resection difficult. 

Poor prognostic factors include a 4th ventricular location, grade 3 tumors (previously known as anaplastic) and incomplete resection. As such, children have a worse prognosis (both 4th ventricular location and anaplastic variant are more common in children). Overall, the 5-year survival rate in children ranges from 50 to 75% 3

Once recurrence has occurred, the prognosis is very poor, with a mortality rate of 90% 3

Rarely extraneural metastases may occur, reported sites include: lymph nodes, mediastinum, lungs, pleura, diaphragmatic muscle, retroperitoneum, liver and bone 14.

The molecular subgroup may have a significant impact on prognosis 10:

  • PFA: poor
  • PFB: good

General imaging differential considerations include:

  • medulloblastoma 
    • similar demographic, especially if around the 4th ventricle
    • arises from vermis
    • less 'plastic', does not tend to extend through foramina
    • enhancement more homogenous
    • calcification less common
  • subependymoma
    • tends to occur in older individuals
    • usually not enhancing
  • choroid plexus papilloma
    • in children usually in the trigone of the lateral ventricles
    • in adults usually in the fourth ventricle (i.e. opposite to ependymoma)
    • more vividly and homogeneously enhancing
    • lacks adjacent parenchymal edema
    • NB choroid plexus carcinoma can be heterogeneous and invade the brain
  • choroid plexus metastasis
    • can appear similar
    • older individuals, usually with a history of malignancy

 

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Cases and figures

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  • Case 7
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  • Case 8: hemorrhagic
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  • Case 12
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