Histopathologic Classification of Childhood Ependymal Tumors
In the most recent World Health Organization (WHO) classification of brain tumors, ependymal tumors are classified into the following four main subtypes:
- Subependymoma (WHO Grade I): A subependymoma is a slow-growing neoplasm, typically attached to the ventricle wall and is composed of glial tumor cell clusters embedded in a fibrillary matrix.
The true incidence of subependymomas (WHO Grade I) is difficult to determine. These tumors are frequently asymptomatic and may be found incidentally at autopsy. Subependymomas probably comprise less than 5% of all ependymal tumors.
- Myxopapillary ependymoma (WHO Grade I): A myxopapillary ependymoma arises almost exclusively in the location of the conus medullaris, cauda equina, and filum terminale of the spinal cord and is characterized histologically by tumor cells arranged in a papillary manner around vascularized myxoid stromal cores.
- Ependymoma (WHO Grade II): The ependymoma, which is considered a Grade II neoplasm originating from the walls of the ventricles or from the spinal canal, is composed of neoplastic ependymal cells. Ependymomas are subdivided, based on histological findings, into the following four subtypes:
- Cellular ependymoma—the most common subtype; usually demonstrates significant cellularity without an increase in mitotic activity.
- Papillary ependymoma—forms linear, epithelial-like surfaces along cerebrospinal fluid exposures.
- Clear cell ependymoma—displays an oligodendroglial-like appearance with perinuclear halos; this variant is preferentially located in the supratentorial compartment of the brain.
- Tanycytic ependymoma—the rarest form of Grade II ependymoma; most commonly found in the spinal cord; tumor cells are arranged in fascicles of variable width and cell density and are poorly intertwined.
- Anaplastic ependymoma (WHO Grade III): Also known as malignant ependymoma. An anaplastic ependymoma is considered a malignant glioma of ependymal differentiation and, compared with the Grade II ependymomas, shows increased cellularity and increased mitotic activity, often associated with microvascular proliferation and necrosis.
In children, approximately 65% to 75% of ependymomas arise in the posterior fossa. Although supratentorial and infratentorial ependymomas are believed to arise from radial glia cells, they have different genomic, gene expression, and immunohistochemical signatures.[2-4] Supratentorial tumors are more often characterized by neuronal differentiation.
Subependymomas and myxopapillary ependymomas are usually considered to be clinically and pathologically distinct from the Grade II and Grade III ependymomas. In Grade II and Grade III ependymomas, the relationship between histological features and survival has varied among studies, although most recent larger studies and meta-analyses have demonstrated that histological grade is an independent predictor of event-free survival.[5-7] A single-institution study suggests that patients with clear-cell ependymomas may have a higher risk of treatment failure than do patients with other forms of WHO Grade II ependymomas; however, confirmation is required in a larger group of unselected patients.
Ependymoblastomas, which generally behave more like medulloblastomas or cerebral neuroectodermal tumors, are considered separate entities from ependymomas and are now classified with the embryonal tumors.[1,5] (Refer to the PDQ summary on Childhood Central Nervous System Embryonal Tumors Treatment for more information.)
The pathologic classification of pediatric brain tumors is a specialized area that is evolving; review of the diagnostic tissue by a neuropathologist who has particular expertise in this area is strongly recommended.
- Louis DN, Ohgaki H, Wiestler OD, et al., eds.: WHO Classification of Tumours of the Central Nervous System. 4th ed. Lyon, France: IARC Press, 2007.
- Taylor MD, Poppleton H, Fuller C, et al.: Radial glia cells are candidate stem cells of ependymoma. Cancer Cell 8 (4): 323-35, 2005. [PUBMED Abstract]
- Andreiuolo F, Puget S, Peyre M, et al.: Neuronal differentiation distinguishes supratentorial and infratentorial childhood ependymomas. Neuro Oncol 12 (11): 1126-34, 2010. [PUBMED Abstract]
- Grill J, Bergthold G, Ferreira C: Pediatric ependymomas: will molecular biology change patient management? Curr Opin Oncol 23 (6): 638-42, 2011. [PUBMED Abstract]
- Louis DN, Ohgaki H, Wiestler OD, et al.: The 2007 WHO classification of tumours of the central nervous system. Acta Neuropathol 114 (2): 97-109, 2007. [PUBMED Abstract]
- Tihan T, Zhou T, Holmes E, et al.: The prognostic value of histological grading of posterior fossa ependymomas in children: a Children's Oncology Group study and a review of prognostic factors. Mod Pathol 21 (2): 165-77, 2008. [PUBMED Abstract]
- Shu HK, Sall WF, Maity A, et al.: Childhood intracranial ependymoma: twenty-year experience from a single institution. Cancer 110 (2): 432-41, 2007. [PUBMED Abstract]
- Fouladi M, Helton K, Dalton J, et al.: Clear cell ependymoma: a clinicopathologic and radiographic analysis of 10 patients. Cancer 98 (10): 2232-44, 2003. [PUBMED Abstract]