Other / Other / MRI

Posterior fossa ependymomas

Posterior fossa ependymomas are the most common ependymoma subtype, typically presenting in children with signs of raised intracranial pressure, ataxia, or acute presentation if hemorrhage occurs. Imaging is performed to characterize mass location, extent, and heterogeneous composition.

Key Images

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Look For First
  • Heterogeneous mass arising from fourth ventricle (lateral recess in PFA, midline floor near obex in PFB)
  • Plastic or toothpaste-like extrusion through foramina of Luschka and Magendie—characteristic feature
  • Mixed solid, cystic, calcified, and hemorrhagic components on both CT and MRI
Key Image Findings
  • CT: coarse calcification present in 50%, cystic areas in 50%, solid component iso- to slightly hyperdense with heterogeneous enhancement and variable hemorrhage
  • MRI T1: solid portions isointense to hypointense relative to white matter
  • MRI T2: tumor hyperintense to white matter, more reliable for tumor margin delineation than non-contrast T1
  • MRI T2* (SWI): blooming artifact from foci of hemorrhage or calcification
  • MRI T1 with gadolinium: heterogeneous enhancement useful for differentiating tumor from vasogenic edema and normal brain
  • DWI/ADC: restricted diffusion may be seen in solid components, especially in anaplastic tumors; interpret with caution in masses with hemorrhage or calcification
  • MR Spectroscopy: elevated choline peak with elevated Cho/Cr ratio reflecting cellularity, reduced NAA, and lipid/lactate rise during degeneration
  • Arising from lateral recess of fourth ventricle (PFA) or midline floor near obex (PFB); PFA tumors more common in infants/young children, PFB in adolescents and adults
Differential Diagnosis
  • Medulloblastoma: similar demographic and fourth-ventricular location, but arises from vermis, less plastic appearance, does not typically extend through foramina, more homogeneous enhancement, calcification less common, and lower ADC values
  • Subependymoma: occurs in older individuals, usually non-enhancing, lack of aggressive growth features
  • Choroid plexus papilloma: in children typically in trigone of lateral ventricles; in adults in fourth ventricle; more vivid and homogeneous enhancement; lacks adjacent parenchymal edema
  • Choroid plexus carcinoma: can occur in infants in lateral ventricles with heterogeneous signal and brain invasion
  • Choroid plexus metastasis: older individuals with history of malignancy, can mimic primary lesions
Discussion

Posterior fossa ependymomas are now classified by DNA-methylation profiling into groups A (poor prognosis, primarily infants/young children) and B (good prognosis, primarily adolescents/adults); H3K27me3 immunohistochemistry (negative in group A, positive in group B) is a useful surrogate when molecular profiling unavailable

The plastic or toothpaste-like appearance with extension through foramina of Luschka and Magendie is a hallmark feature distinguishing ependymomas from other fourth-ventricular tumors

Heterogeneous signal and enhancement on MRI reflects the typical composition of ependymomas with necrosis, calcification, cystic change, and hemorrhage

Prognosis is relatively poor due to surgically challenging fourth-ventricular location; complete resection is difficult, and poor prognostic factors include grade 3 histology and incomplete resection

CSF seeding should be assessed despite being uncommon compared to medulloblastomas; the entire neuraxis should be carefully examined

Extraneural metastases are rare but can occur to lymph nodes, mediastinum, lungs, pleura, liver, bone, and retroperitoneum

Reporting Pearls

Describe the characteristic location (fourth ventricle, arising from lateral recess or midline floor), plastic extension through the foramina of Luschka and Magendie, degree of hydrocephalus, and the heterogeneous nature (solid, cystic, calcification, hemorrhage) on both CT and MRI; note molecular subgroup if available (PFA vs. PFB) as this significantly impacts prognosis.

Pitfalls
  • Confusing ependymoma with medulloblastoma, especially on initial imaging; medulloblastomas are less plastic, do not extend through foramina, and typically have more homogeneous enhancement and lower ADC values
  • Underestimating the extent of tumor on imaging due to heterogeneous composition; careful assessment of plastic extension and foraminal involvement is essential for surgical planning
  • Misinterpreting hemorrhage or calcification-related T2* blooming as diffusion restriction without correlating with ADC maps; in tumors with significant hemorrhage or calcification, DWI must be interpreted cautiously
  • Failure to assess the entire neuraxis for CSF seeding, which although uncommon should not be overlooked
Source: Reference source

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