Brain and spinal tumours affect the Central Nervous System known as the CNS. They come from different types of cells in the brain and are classified according to the cell type and area of the CNS in which they began. Most CNS tumours start in glial cells, the supporting cells of the brain.
Cancerous brain tumours that start in glial cells are known as gliomas and include astrocytomas, ependymomas and oligodendrogliomas. Another group of tumours arise from embryonal cells. These tumours include medulloblastoma and PNET. That is short for primitive neuro-ectodermal tumour. Altogether there are more than 100 different types of brain tumour, most of which are very rare.
Tumours classified as low grade are very slow growing and remain in the part of the brain in which they started.  High grade brain tumours are likely to be more aggressive in spreading into the brain tissue which surrounds them. This can cause pressure and damage to the surrounding areas.
For example, astrocytomas are themselves divided further into four grades:
Grade 1: pilocytic astrocytoma;
Grade 2: diffuse or low grade astrocytoma;
Grade 3: anaplastic astrocytoma; and
Grade 4: glioblastoma.