A 43-year-old woman who had undergone
breast cancer surgery 1 year previously complained of
headache and
nausea. Her brain computed tomography (CT) scan and magnetic resonance imaging (MRI) showed a well-circumscribed, heterogeneously enhanced
tumor in the right thalamus. She underwent gross total resection of the
tumor followed by
radiochemotherapy, and her
clinical course was uneventful after surgery. Histological examination revealed a moderate number of
tumor cells with fine bipolar processes in a mucoid matrix, which suggested
pilocytic astrocytoma. The
tumor was associated with microvascular proliferation but did not show significant mitosis or
necrosis. In some areas, it had an epithelioid appearance, with ribbon-like, cribriform, and pseudoglandular patterns involving cuboid-shaped cells showing nuclear atypia and mitotic figures. Immunohistochemically, the
tumor cells were positive for
glial fibrillary acidic protein (GFAP) and
vimentin in the area resembling
pilocytic astrocytoma, but in the epithelioid area they were negative for GFAP and
vimentin as well as for
breast cancer markers, including AE1/AE3. The proliferating potential, represented by the MIB-1 labeling index, was high (82.5%) in the area of epithelioid appearance, compared to only 3% in the area of
pilocytic astrocytoma-like appearance. As a rare histoarchitectural variant of
glioblastoma, the epithelioid pattern may represent a very primitive
tumor cell phenotype. Typically, this pattern is characterized by well-circumscribed masses, although its clinical significance is unknown.