Most important effect - surrounding edema, with subsequent swelling of white matter. This sometimes responds to steroids or hyperosmolar urea. A tumor of arachnoid cells, especially those within the arachnoidal villi. These are well circumscribed benign tumors 4570 which are attached to and frequently invade the dura. They cause symptoms by compression of the adjacent brain or by invasion of the skull, which commonly shows a reactive hyperostosis.
Malignant change is very rare and manifests histologically as brain invasion.
Over time, the low grade astrocytomas may progress to the anaplastic variant, which in turn may progress to glioblastoma multiforme. These tumors occur primarily in the cerebral hemispheres. They are composed of a uniform population of astrocytes in a fibrillary background. The astrocytes show variable atypia and low grade astrocytomas show minimal hypercellularity and lack mitoses. They are often difficult to differentiate from reactive gliosis.