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. This is "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. At least 50% of GBM's arise from a pre-existing lower grade lesion.