Posterior fossa ependymoma
General description
Posterior fossa ependymomas are divided into two principal molecular subgroups, designated Group A (PFA) and Group B (PFB), each defined by distinct epigenetic, transcriptional, and cytogenetic profiles.
Posterior fossa ependymoma Group A tumors present almost exclusively in early childhood, with a median age at diagnosis of approximately 2 years and more than 95 percent of cases occurring before 6 years of age; they exhibit a slight male predominance and account for the majority of posterior fossa ependymomas in infants and young children. In contrast, Group B tumors arise predominantly in adolescents and adults, with a median age around 20 years, fewer than 5 percent presenting before age 5, and up to 90 percent occurring in patients older than 18 years; these lesions show a modest female predominance relative to Group A.
References
- Leclerc, Thomas, et al. "Imaging features to distinguish posterior fossa ependymoma subgroups." European Radiology 34.3 (2024): 1534-1544.
Posterior fossa mass
The vast majority of posterior fossa ependymomas arise adjacent to or within the fourth ventricle, often extending through the foramina of Luschka and Magendie.
Common locations include:
- The lateral recess of the fourth ventricle (predominantly PFA subgroup)
- The midline floor of the fourth ventricle near the obex (predominantly PFB subgroup)
Rare intraparenchymal (ectopic) or extra‐axial ependymomas in the cerebellar parenchyma or cisternal spaces have been reported.
Posterior fossa ependymomas generally have well-defined but lobulated margins.
The solid component of the tumor appears iso- to slightly hyperdense relative to the adjacent cerebellar parenchyma on CT. On T1-weighted images, it is isointense to hypointense compared to white matter, while on T2-weighted images, it appears hyperintense. Contrast enhancement is heterogeneous in more than 90% of PFA tumors, whereas homogeneous enhancement is more commonly seen in PFB tumors (75%). Restricted diffusion may be seen in the solid components, especially in higher-grade or anaplastic tumors.
Calcification is present in approximately 50% of cases, often appearing coarse. Cystic areas are also observed in about 50% of cases and may enlarge over time. Hemorrhage is variable but less common than calcification.