Chondro-Osseous Metaplasia in an Anaplastic Ependymoma: Diagnostic Challenges in an Uncommon Tumor Phenotype

A bstract Ependymomas commonly arise in the posterior fossa in children and typically display classical glial architecture. However, true mesenchymal metaplasia in the form of cartilaginous or osseous differentiation is exceedingly rare and may pose significant diagnostic challenges. We report a rare case of a posterior fossa ependymoma with anaplastic histology, exhibiting chondro-osseous metaplasia in an 18-month-old girl. The patient presented with recurrent vomiting, seizures, headache, and visual disturbance. Magnetic resonance imaging revealed a heterogeneously enhancing posterior fossa mass arising from the fourth ventricle with inferior extension to the C2 level. Near-total surgical excision was performed. Histopathological examination demonstrated a hypercellular tumor composed of glial cells arranged in classical perivascular pseudorosettes and occasional true ependymal rosettes, with microvascular proliferation and necrosis. Notably, multiple well-formed foci of mature cartilage and bone were identified within the tumor, confirming chondro-osseous metaplasia. This rare histological variant can mimic other matrix-producing posterior fossa neoplasms, potentially leading to misdiagnosis. Recognition of classical ependymal features is therefore critical for accurate classification. Reporting such uncommon variants expands the morphological spectrum of ependymomas and underscores the importance of meticulous histopathological evaluation.