DIAGNOSIS: Cerebellar low grade astroblastoma
Astroblastoma is a rare glial neoplasm of uncertain histogenesis. Most often seen in young adults, it usually occurs in the cerebral hemispheres, often shows cortical location and may extend into the subarachnoid space (4, 7). Examples arising from the cerebellar cortex have also been reported, but they are exceedingly rare (3, 4). Radiologically, they are well-demarcated from the surrounding brain. They often show diffuse enhancement after contrast administration and, not infrequently, contain cysts (8). The histological hallmark is the characteristic "cartwheel" perivascular pseudorosette. Originally described by Bailey and Bucy (3), it is composed of neoplastic cells having thick and short cytoplasmic processes and peripheral nuclei radiating from a vessel. When they become less cohesive astroblastic pseudorosettes may appear as true papillary structures. Features of the utmost diagnostic importance are also the lack of fibrillary background and compressive rather than infiltrative margin (this latter feature being seen in both low and high grade tumors. A prominent vascular network with a variable number of hyalinized vessels is also a frequent feature of astroblastoma (4, 5). Immunoreactivity for GFAP and S-100 protein is the rule, but the number of positive cells varies from case to case and even from different areas of the same tumor (5). Some astroblastomas may show focal expression of EMA (5, 8). Immunoreactivity for NSE has been reported, but it seems to be more likely the result of a non-specific reaction (9). By electron microscopy, astroblastomas have distinct blood vessels with thick basement membrane and abundant collagen. The processes of cells that constitute pseudorosettes connect with to collagen fibers without intervening basal lamina (6, 9, 10). A recent study demonstrated that astroblastomas have characteristic chromosomal aberrations as they show gain of chromosome 19 and 20. These peculiar chromosomal abnormalities sustain the view that astroblastoma is a distinct entity rather than a variant of ependymoma (5).
Prognosis of astroblastoma is variable and depends on extent of resection and grade (4, 11). Although not accepted by the current WHO classification of brain tumors (7), a grading system has been proposed (11). Based on degree of cellular atypia, mitoses, and microvascular proliferation, this grading system reportedly allows distinction between low and high grade tumors. Low grade lesions have indolent behavior and excellent long term survival after total resection whereas high grade astroblastomas are aggressive and often convert into a glioblastoma (4, 11).
Differential diagnosis includes ependymoma, particularly supratentorial hemispheric and cortical examples, astrocytoma with focal astroblastic-like pseudorosettes and papillary meningioma. Unlike astroblastoma, ependymoma has fibrillary background (5) and ultrastructurally shows more frequently cilia and intracellular lumina with microvillous projections (6, 8). Anaplastic astrocytomas with astroblastic pseudorosettes always infiltrate the surrounding brain (5). Papillary meningioma can be confused with astroblastoma because of superficial location, but unlike astroblastoma it shows no expression of GFAP (2) and, ultrastructurally, cell-to-cell junctions consist of well-formed desmosomes (1).
Contributed by Iqroop Chopra, MD MRCS, Federico Roncaroli, MD, Vasileios Apostolopoulos MD MRCS, Jilly Moss FRCPath, David Peston FIBMS, Kevin O’Neill MD FRCS, FRCS-SN