The proposed cell of origin generally forms the basis for the classification of primary brain tumors, a practice highlighted by Bailey and Cushing in 1926 (1) and continued to the present time (2, 3). Despite the long history of the naming of tumors, “new” tumors continue to be identified when their correct cell of origin is identified by additional studies or modern technology. The cerebral neurocytoma typifies such a tumor whose histogenesis from neuronal lines rather than from oligodendroglial lines was recognized when electron microscopy identified tumor cell synapses. The advent of molecular biology has ushered in a new dimension to tumor classification, or reclassification, most notably in its current potential to distinguish between chemosensitive and chemoresistant infiltrating gliomas.
In 1999, Tihan et al (4) reclassified a subset of previously diagnosed pilocytic astrocytomas of the suprasellar-hypothalamic region on the basis of their more aggressive clinical course, monomorphous histology, myxoid features, and an absence of Rosenthal fibers. They termed these tumors “pediatric astrocytomas with monomorphous pilomyxoid features.” Three additional cases were subsequently described by Fuller et al (5) and included the immunophenotype and electron microscopic appearance of these neoplasms. Because of the ultrastructural appearance was similar to the periventricular tanycyte, Fuller et al suggested a tanycytic origin for these tumors, although they concluded that current studies were insufficient to redefine this unique group of third ventricular gliomas as “tanycytomas.”
In the present issue of the AJNR, Lieberman et al more fully detail the neuroradiologic features of Fuller’s cases and add two additional cases with similar histology, immunophenotype, and ultrastructural features.
Are the available data now sufficient to classify these tumors as tanycytomas? Probably not. Location and ultrastructure are suggestive of tanycytic origin, but as Fuller et al have pointed out, they are insufficient for a definite statement regarding the histogenesis of this neoplasm. Additional immunohistochemical studies to examine other tanycytic antigens such as macrophage migration inhibitory factor (6) or a tanycytic-specific antigen such as P5 (7) would strengthen the hypothesis of a tanycytic origin of this suprasellar pilomyxoid tumor. The potential role of molecular biology in identifying the cell of origin is, or course, uncertain, but it has been helpful in distinguishing among the different glial tumors such as astrocytoma, oligodendroglioma, and ependymoma (2) as well as predicting biologic behavior (8). Classification as a “suprasellar pilomyxoid glioma” might be a compromise term until its cell of origin is established.
Sato et al (9) recently propose a tanycytic origin for the choroid glioma (9), a well-circumscribed third ventricular neoplasm. Its histology and more benign clinical course (10) separate it from the suprasellar pilomyxoid glioma and its molecular profile displays an absence of genetic mutations commonly associated with gliomas (11). Currently, there is insufficient information to determine links between the low grade choroid glioma with the more aggressive suprasellar pilomyxoid glioma.
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