Pilocytic astrocytoma

Definition

A rare astrocytoma characterized by bipolar hair-like projections extending from the neoplastic astrocytes, myxoid areas, eosinophilic granular bodies, and Rosenthal fibers. This WHO grade 1 tumor is most often located in the cerebellum in children, or in the hypothalamus, brain stem, optic chiasma, and brain hemispheres in adults. Depending on the primary location and the growth rate of the tumor, patients may present with signs of increased intracranial pressure or focal neurological signs such as decreased visual acuity, ataxia, and/or nystagmus, among other symptoms. Molecularly, it exhibits alterations in genes of the MAPK signaling pathway, most commonly KIAA1549::BRAF fusion.