Challenging anatomical paradigms: unexpected supratentorial localization of posterior fossa B ependymomas identified by DNA methylation.

Molecular subgroups of ependymomas are strongly associated with their primary anatomical compartments, namely the supratentorial region, posterior fossa and spine locations. Although it is generally accepted that posterior fossa ependymomas may arise from radial glia-like progenitor cells, the precise cellular origin of the distinct posterior fossa molecular subgroups remains to be determined. Given that DNA methylation profile strongly recapitulates developmental lineage and the cell of origin, the characterization of unexpectedly located molecular subgroups of ependymomas may provide insights into their cells of origin and potential migratory dynamics during embryogenesis and brain development. Herein, we describe two male school-aged pediatric patients with supratentorial tumors located in the third ventricle/pineal region that were classified as posterior fossa group B ependymomas based on DNA methylation profiling. To our knowledge, this represents the first reports of posterior fossa group B ependymomas identified outside the posterior fossa compartment.