Photomicrograph from a contrast-enhancing hypothalamic mass in a 4-year-old boy. The histopathologic features consist of a loosely textured neoplasm containing Rosenthal fibers (arrow) and stellate astrocytes. Typically, the loosely textured spongy tissue alternates with more compact areas, resulting in a biphasic architecture.

Pilocytic astrocytomas are circumscribed and often cystic astrocytomas that are common in children and young adults. Although they can occur anywhere along the neuraxis, including the cerebral hemispheres and spinal cord, the cerebellum, optic nerve, and hypothalamus are favored. Pilocytic astrocytomas may contain microvascular proliferation, nuclear atypia, and an occasional mitotic figure. Despite the fact that these morphologic features are ominous findings in "diffuse" astrocytomas, they do not appear to affect prognosis adversely in pilocytic astrocytomas. Malignant degeneration has been reported, but it is rare [63][67][68].