Glioblastomas (also called GBM) are malignant Grade IV tumors, where a large portion of tumor cells are reproducing and dividing at any given time. They are nourished by an ample and abnormal tumor vessel blood supply. The tumor is predominantly made up of abnormal astrocytic cells, but also contain a mix of different cell types (including blood vessels) and areas of dead cells (necrosis). Glioblastomas are infiltrative and invade into nearby regions of the brain. They can also sometimes spread to the opposite side of the brain through connection fibers (corpus callosum). It is exceedingly rare for glioblastomas to spread outside of the brain.
Glioblastomas may arise de novo, meaning they begin as a Grade IV tumor with no evidence of a lower grade precursor. De novo tumors are the most common form of glioblastoma and tend to be more aggressive and tend to affect older patients. Alternatively, secondary glioblastomas may progress from a lower-grade astrocytic tumors (Grade II or Grade III) and evolve into Grade IV tumors over time. In general, these tumors tend to be slower growing initially, but can progressively become aggressive.
Glioblastomas are usually diagnosed as either IDH-wildtype or IDH-mutant (please see the Genetic Profile section below for more information). IDH-wildtype glioblastomas are more common, tend to be more aggressive, and have worse prognosis than IDH-mutant glioblastomas.