Coronal gross brain section. There is gross expansion of the left cerebral hemisphere. This is caused by infiltration of a diffuse low-grade (WHO Grade 2) fibrillary astrocytoma. Most of this infiltration has occurred nondestructively.  This is similar to the concept of "gliomatosis cerebri". In addition, the superior surface of the expanded hemisphere has a discoloration from a local area of transformation from Grade 2 into a Grade 4 astroctyoma (Glioblastoma Multiforme or GBM). This is a natural progression, from low-grade to high-grade. Approximately 30 - 50% of GBM's are thought to arise from a pre-existing lower grade astrocytoma (WHO Gr 2 or 3).