False-colour computed tomography (CT) brain scan revealing a glioblastoma, a tumour derived from the astrocytes, the red-coloured mass filling a large proportion of the brain hemisphere at right of this coronal image. Glioblastomas & astrocytomas (a related tumour type), the most frequent of all brain tumours, are derived from cells that support the nerve tissue. Their degree of malignancy varies considerably and is graded from I to IV, from low to highly malignant respectively.