Newly codified glial neoplasms of the 2007 WHO Classification of Tumours of the Central Nervous System: angiocentric glioma, pilomyxoid astrocytoma and pituicytoma

Abstract

The 4(th) edition of the WHO Classification of Tumours of the Nervous System (WHO 2007) introduces changes that reflect both the recognition of new brain tumour types and a better understanding of neoplastic behavior. Three new tumours, angiocentric glioma (AG), pilomyxoid astrocytoma (PMA), and pituicytoma are added to the section on gliomas. AG is a slowly growing cerebral tumour that typically presents with seizures in children and young adults. It is characterized by monomorphous, bipolar tumour cells with a striking perivascular growth pattern. Although the 'cell of origin' of AG is not clear, ultrastructural evidence points to an ependymal derivation. Typically, AG can be cured by total resection, and is designated WHO grade I. PMA is a solid, circumscribed tumour occurring mainly in the hypothalamic region of young children. It is composed of a monomorphous population of bipolar tumour cells within a rich myxoid background, with a conspicuous anglocentric arrangement. While PMA is considered a more aggressive variant of pilocytic astrocytoma, this relationship awaits further clarification. The PMA has been designated WHO grade II. The pituicytoma, involves the posterior pituitary and/or its stalk and affects adults. It is solid in architecture, composed of spindle cells and presumably derived from pituicytes. Pituicytomas are indolent tumours, and are designated WHO grade I.

Figure 1

Angiocentric glioma (AG). Axial fluid attenuation inversion recovery image of AG in a young adult male. The lesion partially expands the cortex, and is hyperintense on T2‐weighted and hypointense on T1‐weighted images. This lesion did not exhibit contrast enhancement.

Figure 2

Angiocentric glioma (AG). AG exhibits solid and perivascular growth patterns (A). The latter is seen at higher power in (B). In addition to solid and perivascular growth, a highly characteristic feature is diffuse infiltration (C). In some areas, the perivascular growth takes the form of perivascular pseudorosettes, similar to those seen in ependymoma (D).

Figure 3

Pilomyxoid astrocytoma. Axial, contrast‐enhanced T1‐weighted image of pilomyxoid astrocytoma in a 2‐year‐old girl. The tumor demonstrates avid enhancement as well as central hypointense areas.

Figure 4

Pilomyxoid astrocytoma. Characteristic histologic features include an overall monomorphic appearance, prominent myxoid microcystic background (A), spindled astrocytes with delicate, elongated cytoplasmic processes (B), and perivascular pseudorosettes (C,D).

Figure 5

Pituicytoma. The histology varies from streaming bundles of cells to a somewhat storiform pattern (A). Strong staining for glial fibrillary acidic protein is seen (B).